diff --git a/scripts/motan/analyzers.py b/scripts/motan/analyzers.py
new file mode 100644
index 00000000..9e3f3b57
--- /dev/null
+++ b/scripts/motan/analyzers.py
@@ -0,0 +1,195 @@
+# Log data analyzing functions
+#
+# Copyright (C) 2021  Kevin O'Connor <kevin@koconnor.net>
+#
+# This file may be distributed under the terms of the GNU GPLv3 license.
+import collections
+
+
+######################################################################
+# Analysis code
+######################################################################
+
+# Analyzer handlers: {name: class, ...}
+AHandlers = {}
+
+# Calculate a derivative (position to velocity, or velocity to accel)
+class GenDerivative:
+    DataSets = [
+        ('derivative:<dataset>', 'Derivative of the given dataset'),
+    ]
+    def __init__(self, amanager, params):
+        self.amanager = amanager
+        self.source = params
+        amanager.setup_dataset(self.source)
+    def get_label(self):
+        label = self.amanager.get_label(self.source)
+        lname = label['label']
+        units = label['units']
+        if '(mm)' in units:
+            rep = [('Position', 'Velocity'), ('(mm)', '(mm/s)')]
+        elif '(mm/s)' in units:
+            rep = [('Velocity', 'Acceleration'), ('(mm/s)', '(mm/s^2)')]
+        else:
+            return {'label': 'Derivative', 'units': 'Unknown'}
+        for old, new in rep:
+            lname = lname.replace(old, new).replace(old.lower(), new.lower())
+            units = units.replace(old, new).replace(old.lower(), new.lower())
+        return {'label': lname, 'units': units}
+    def generate_data(self):
+        inv_seg_time = 1. / self.amanager.get_segment_time()
+        data = self.amanager.get_datasets()[self.source]
+        deriv = [(data[i+1] - data[i]) * inv_seg_time
+                 for i in range(len(data)-1)]
+        return [deriv[0]] + deriv
+AHandlers["derivative"] = GenDerivative
+
+# Calculate a kinematic stepper position from the toolhead requested position
+class GenKinematicPosition:
+    DataSets = [
+        ('kin:<stepper>', 'Stepper position derived from toolhead kinematics'),
+    ]
+    def __init__(self, amanager, params):
+        self.amanager = amanager
+        status = self.amanager.get_initial_status()
+        kin = status['configfile']['settings']['printer']['kinematics']
+        if kin not in ['cartesian', 'corexy']:
+            raise amanager.error("Unsupported kinematics '%s'" % (kin,))
+        if params not in ['stepper_x', 'stepper_y', 'stepper_z']:
+            raise amanager.error("Unknown stepper '%s'" % (params,))
+        if kin == 'corexy' and params in ['stepper_x', 'stepper_y']:
+            self.source1 = 'trapq:toolhead:x'
+            self.source2 = 'trapq:toolhead:y'
+            if params == 'stepper_x':
+                self.generate_data = self.generate_data_corexy_plus
+            else:
+                self.generate_data = self.generate_data_corexy_minus
+            amanager.setup_dataset(self.source1)
+            amanager.setup_dataset(self.source2)
+        else:
+            self.source1 = 'trapq:toolhead:' + params[-1:]
+            self.source2 = None
+            self.generate_data = self.generate_data_passthrough
+            amanager.setup_dataset(self.source1)
+    def get_label(self):
+        return {'label': 'Position', 'units': 'Position\n(mm)'}
+    def generate_data_corexy_plus(self):
+        datasets = self.amanager.get_datasets()
+        data1 = datasets[self.source1]
+        data2 = datasets[self.source2]
+        return [d1 + d2 for d1, d2 in zip(data1, data2)]
+    def generate_data_corexy_minus(self):
+        datasets = self.amanager.get_datasets()
+        data1 = datasets[self.source1]
+        data2 = datasets[self.source2]
+        return [d1 - d2 for d1, d2 in zip(data1, data2)]
+    def generate_data_passthrough(self):
+        return self.amanager.get_datasets()[self.source1]
+AHandlers["kin"] = GenKinematicPosition
+
+# Calculate a position deviation
+class GenDeviation:
+    DataSets = [
+        ('deviation:<dataset1>-<dataset2>', 'Difference between datasets'),
+    ]
+    def __init__(self, amanager, params):
+        self.amanager = amanager
+        parts = params.split('-')
+        if len(parts) != 2:
+            raise amanager.error("Invalid deviation '%s'" % (params,))
+        self.source1, self.source2 = parts
+        amanager.setup_dataset(self.source1)
+        amanager.setup_dataset(self.source2)
+    def get_label(self):
+        label1 = self.amanager.get_label(self.source1)
+        label2 = self.amanager.get_label(self.source2)
+        if label1['units'] != label2['units']:
+            return {'label': 'Deviation', 'units': 'Unknown'}
+        parts = label1['units'].split('\n')
+        units = '\n'.join([parts[0]] + ['Deviation'] + parts[1:])
+        return {'label': label1['label'] + ' deviation', 'units': units}
+    def generate_data(self):
+        datasets = self.amanager.get_datasets()
+        data1 = datasets[self.source1]
+        data2 = datasets[self.source2]
+        return [d1 - d2 for d1, d2 in zip(data1, data2)]
+AHandlers["deviation"] = GenDeviation
+
+
+######################################################################
+# List datasets
+######################################################################
+
+def list_datasets():
+    datasets = []
+    for ah in sorted(AHandlers.keys()):
+        datasets += AHandlers[ah].DataSets
+    return datasets
+
+
+######################################################################
+# Data generation
+######################################################################
+
+# Manage raw and generated data samples
+class AnalyzerManager:
+    error = None
+    def __init__(self, lmanager, segment_time):
+        self.lmanager = lmanager
+        self.error = lmanager.error
+        self.segment_time = segment_time
+        self.raw_datasets = collections.OrderedDict()
+        self.gen_datasets = collections.OrderedDict()
+        self.datasets = {}
+        self.dataset_times = []
+        self.duration = 5.
+    def set_duration(self, duration):
+        self.duration = duration
+    def get_segment_time(self):
+        return self.segment_time
+    def get_datasets(self):
+        return self.datasets
+    def get_dataset_times(self):
+        return self.dataset_times
+    def get_initial_status(self):
+        return self.lmanager.get_initial_status()
+    def setup_dataset(self, name):
+        name = name.strip()
+        if name in self.raw_datasets:
+            return self.raw_datasets[name]
+        if name in self.gen_datasets:
+            return self.gen_datasets[name]
+        nparts = name.split(':')
+        if nparts[0] in self.lmanager.available_dataset_types():
+            hdl = self.lmanager.setup_dataset(name)
+            self.raw_datasets[name] = hdl
+        else:
+            cls = AHandlers.get(nparts[0])
+            if cls is None:
+                raise self.error("Unknown dataset '%s'" % (name,))
+            hdl = cls(self, ':'.join(nparts[1:]))
+            self.gen_datasets[name] = hdl
+        self.datasets[name] = []
+        return hdl
+    def get_label(self, dataset):
+        hdl = self.raw_datasets.get(dataset)
+        if hdl is None:
+            hdl = self.gen_datasets.get(dataset)
+            if hdl is None:
+                raise error("Unknown dataset '%s'" % (dataset,))
+        return hdl.get_label()
+    def generate_datasets(self):
+        # Generate raw data
+        list_hdls = [(self.datasets[name], hdl)
+                     for name, hdl in self.raw_datasets.items()]
+        initial_start_time = self.lmanager.get_initial_start_time()
+        start_time = t = self.lmanager.get_start_time()
+        end_time = start_time + self.duration
+        while t < end_time:
+            t += self.segment_time
+            self.dataset_times.append(t - initial_start_time)
+            for dl, hdl in list_hdls:
+                dl.append(hdl.pull_data(t))
+        # Generate analyzer data
+        for name, hdl in self.gen_datasets.items():
+            self.datasets[name] = hdl.generate_data()
diff --git a/scripts/motan/motan_graph.py b/scripts/motan/motan_graph.py
new file mode 100755
index 00000000..b593c1ce
--- /dev/null
+++ b/scripts/motan/motan_graph.py
@@ -0,0 +1,144 @@
+#!/usr/bin/env python
+# Script to perform motion analysis and graphing
+#
+# Copyright (C) 2019-2021  Kevin O'Connor <kevin@koconnor.net>
+#
+# This file may be distributed under the terms of the GNU GPLv3 license.
+import sys, optparse, ast
+import matplotlib
+import readlog, analyzers
+try:
+    import urlparse
+except:
+    import urllib.parse as urlparse
+
+
+######################################################################
+# Graphing
+######################################################################
+
+def plot_motion(amanager, graphs):
+    # Generate data
+    for graph in graphs:
+        for dataset, plot_params in graph:
+            amanager.setup_dataset(dataset)
+    amanager.generate_datasets()
+    datasets = amanager.get_datasets()
+    times = amanager.get_dataset_times()
+    # Build plot
+    fontP = matplotlib.font_manager.FontProperties()
+    fontP.set_size('x-small')
+    fig, rows = matplotlib.pyplot.subplots(nrows=len(graphs), sharex=True)
+    if len(graphs) == 1:
+        rows = [rows]
+    rows[0].set_title("Motion Analysis")
+    for graph, graph_ax in zip(graphs, rows):
+        graph_units = graph_twin_units = twin_ax = None
+        for dataset, plot_params in graph:
+            label = amanager.get_label(dataset)
+            ax = graph_ax
+            if graph_units is None:
+                graph_units = label['units']
+                ax.set_ylabel(graph_units)
+            elif label['units'] != graph_units:
+                if graph_twin_units is None:
+                    ax = twin_ax = graph_ax.twinx()
+                    graph_twin_units = label['units']
+                    ax.set_ylabel(graph_twin_units)
+                elif label['units'] == graph_twin_units:
+                    ax = twin_ax
+                else:
+                    graph_units = "Unknown"
+                    ax.set_ylabel(graph_units)
+            pparams = {'label': label['label'], 'alpha': 0.8}
+            pparams.update(plot_params)
+            ax.plot(times, datasets[dataset], **pparams)
+            ax.legend(loc='best', prop=fontP)
+            ax.grid(True)
+    rows[-1].set_xlabel('Time (s)')
+    return fig
+
+
+######################################################################
+# Startup
+######################################################################
+
+def setup_matplotlib(output_to_file):
+    global matplotlib
+    if output_to_file:
+        matplotlib.use('Agg')
+    import matplotlib.pyplot, matplotlib.dates, matplotlib.font_manager
+    import matplotlib.ticker
+
+def parse_graph_description(desc):
+    if '?' not in desc:
+        return (desc, {})
+    dataset, params = desc.split('?', 1)
+    params = {k: v for k, v in urlparse.parse_qsl(params)}
+    for fkey in ['alpha']:
+        if fkey in params:
+            params[fkey] = float(params[fkey])
+    return (dataset, params)
+
+def list_datasets():
+    datasets = readlog.list_datasets() + analyzers.list_datasets()
+    out = ["\nAvailable datasets:\n"]
+    for dataset, desc in datasets:
+        out.append("%-24s: %s\n" % (dataset, desc))
+    out.append("\n")
+    sys.stdout.write("".join(out))
+    sys.exit(0)
+
+def main():
+    # Parse command-line arguments
+    usage = "%prog [options] <logname>"
+    opts = optparse.OptionParser(usage)
+    opts.add_option("-o", "--output", type="string", dest="output",
+                    default=None, help="filename of output graph")
+    opts.add_option("-s", "--skip", type="float", default=0.,
+                    help="Set the start time to graph")
+    opts.add_option("-d", "--duration", type="float", default=5.,
+                    help="Number of seconds to graph")
+    opts.add_option("--segment-time", type="float", default=0.000100,
+                    help="Analysis segment time (default 0.000100 seconds)")
+    opts.add_option("-g", "--graph", help="Graph to generate (python literal)")
+    opts.add_option("-l", "--list-datasets", action="store_true",
+                    help="List available datasets")
+    options, args = opts.parse_args()
+    if options.list_datasets:
+        list_datasets()
+    if len(args) != 1:
+        opts.error("Incorrect number of arguments")
+    log_prefix = args[0]
+
+    # Open data files
+    lmanager = readlog.LogManager(log_prefix)
+    lmanager.setup_index()
+    lmanager.seek_time(options.skip)
+    amanager = analyzers.AnalyzerManager(lmanager, options.segment_time)
+    amanager.set_duration(options.duration)
+
+    # Default graphs to draw
+    graph_descs = [
+        ["trapq:toolhead:velocity?color=green"],
+        ["trapq:toolhead:accel?color=green"],
+        ["deviation:stepq:stepper_x-kin:stepper_x?color=blue"],
+    ]
+    if options.graph is not None:
+        graph_descs = ast.literal_eval(options.graph)
+    graphs = [[parse_graph_description(g) for g in graph_row]
+              for graph_row in graph_descs]
+
+    # Draw graph
+    setup_matplotlib(options.output is not None)
+    fig = plot_motion(amanager, graphs)
+
+    # Show graph
+    if options.output is None:
+        matplotlib.pyplot.show()
+    else:
+        fig.set_size_inches(8, 6)
+        fig.savefig(options.output)
+
+if __name__ == '__main__':
+    main()
diff --git a/scripts/motan/readlog.py b/scripts/motan/readlog.py
new file mode 100644
index 00000000..21f4485c
--- /dev/null
+++ b/scripts/motan/readlog.py
@@ -0,0 +1,311 @@
+# Code for reading data logs produced by data_logger.py
+#
+# Copyright (C) 2021  Kevin O'Connor <kevin@koconnor.net>
+#
+# This file may be distributed under the terms of the GNU GPLv3 license.
+import json, zlib
+
+class error(Exception):
+    pass
+
+
+######################################################################
+# Log data handlers
+######################################################################
+
+# Log data handlers: {name: class, ...}
+LogHandlers = {}
+
+# Extract requested position, velocity, and accel from a trapq log
+class HandleTrapQ:
+    ParametersSubscriptionId = 2
+    ParametersMin = ParametersMax = 3
+    DataSets = [
+        ('trapq:<name>:velocity', 'Requested velocity for the given trapq'),
+        ('trapq:<name>:<axis>', 'Requested axis (x, y, or z) position'),
+        ('trapq:<name>:<axis>_velocity', 'Requested axis velocity'),
+        ('trapq:<name>:<axis>_accel', 'Requested axis acceleration'),
+    ]
+    def __init__(self, lmanager, name):
+        self.name = name
+        self.jdispatch = lmanager.get_jdispatch()
+        self.cur_data = [(0., 0., 0., 0., (0., 0., 0.), (0., 0., 0.))]
+        self.data_pos = 0
+        tq, trapq_name, datasel = name.split(':')
+        ptypes = {}
+        ptypes['velocity'] = {
+            'label': '%s velocity' % (trapq_name,),
+            'units': 'Velocity\n(mm/s)', 'func': self._pull_velocity
+        }
+        ptypes['accel'] = {
+            'label': '%s acceleration' % (trapq_name,),
+            'units': 'Acceleration\n(mm/s^2)', 'func': self._pull_accel
+        }
+        for axis, name in enumerate("xyz"):
+            ptypes['%s' % (name,)] = {
+                'label': '%s %s position' % (trapq_name, name), 'axis': axis,
+                'units': 'Position\n(mm)', 'func': self._pull_axis_position
+            }
+            ptypes['%s_velocity' % (name,)] = {
+                'label': '%s %s velocity' % (trapq_name, name), 'axis': axis,
+                'units': 'Velocity\n(mm/s)', 'func': self._pull_axis_velocity
+            }
+            ptypes['%s_accel' % (name,)] = {
+                'label': '%s %s acceleration' % (trapq_name, name),
+                'axis': axis, 'units': 'Acceleration\n(mm/s^2)',
+                'func': self._pull_axis_accel
+            }
+        pinfo = ptypes.get(datasel)
+        if pinfo is None:
+            raise error("Unknown trapq data selection '%s'" % (datasel,))
+        self.label = {'label': pinfo['label'], 'units': pinfo['units']}
+        self.axis = pinfo.get('axis')
+        self.pull_data = pinfo['func']
+    def get_label(self):
+        return self.label
+    def _find_move(self, req_time):
+        data_pos = self.data_pos
+        while 1:
+            move = self.cur_data[data_pos]
+            print_time, move_t, start_v, accel, start_pos, axes_r = move
+            if req_time <= print_time + move_t:
+                return move, req_time >= print_time
+            data_pos += 1
+            if data_pos < len(self.cur_data):
+                self.data_pos = data_pos
+                continue
+            jmsg = self.jdispatch.pull_msg(req_time, self.name)
+            if jmsg is None:
+                return move, False
+            self.cur_data = jmsg['data']
+            self.data_pos = data_pos = 0
+    def _pull_axis_position(self, req_time):
+        move, in_range = self._find_move(req_time)
+        print_time, move_t, start_v, accel, start_pos, axes_r = move
+        mtime = max(0., min(move_t, req_time - print_time))
+        dist = (start_v + .5 * accel * mtime) * mtime;
+        return start_pos[self.axis] + axes_r[self.axis] * dist
+    def _pull_axis_velocity(self, req_time):
+        move, in_range = self._find_move(req_time)
+        if not in_range:
+            return 0.
+        print_time, move_t, start_v, accel, start_pos, axes_r = move
+        return (start_v + accel * (req_time - print_time)) * axes_r[self.axis]
+    def _pull_axis_accel(self, req_time):
+        move, in_range = self._find_move(req_time)
+        if not in_range:
+            return 0.
+        print_time, move_t, start_v, accel, start_pos, axes_r = move
+        return accel * axes_r[self.axis]
+    def _pull_velocity(self, req_time):
+        move, in_range = self._find_move(req_time)
+        if not in_range:
+            return 0.
+        print_time, move_t, start_v, accel, start_pos, axes_r = move
+        return start_v + accel * (req_time - print_time)
+    def _pull_accel(self, req_time):
+        move, in_range = self._find_move(req_time)
+        if not in_range:
+            return 0.
+        print_time, move_t, start_v, accel, start_pos, axes_r = move
+        return accel
+LogHandlers["trapq"] = HandleTrapQ
+
+# Extract positions from queue_step log
+class HandleStepQ:
+    ParametersSubscriptionId = 2
+    ParametersMin = ParametersMax = 2
+    DataSets = [
+        ('stepq:<stepper>', 'Commanded position of the given stepper'),
+    ]
+    def __init__(self, lmanager, name):
+        self.name = name
+        self.jdispatch = lmanager.get_jdispatch()
+        self.step_data = [(0., 0.), (0., 0.)] # [(time, pos), ...]
+        self.data_pos = 0
+    def get_label(self):
+        label = '%s position' % (self.name.split(':')[1],)
+        return {'label': label, 'units': 'Position\n(mm)'}
+    def pull_data(self, req_time):
+        while 1:
+            data_pos = self.data_pos
+            step_data = self.step_data
+            next_time, next_pos = step_data[data_pos + 1]
+            if req_time < next_time:
+                return step_data[data_pos][1]
+            if data_pos + 2 < len(step_data):
+                self.data_pos = data_pos + 1
+                continue
+            self._pull_block(req_time)
+    def _pull_block(self, req_time):
+        step_data = self.step_data
+        del step_data[:-1]
+        self.data_pos = 0
+        # Read data block containing requested time frame
+        while 1:
+            jmsg = self.jdispatch.pull_msg(req_time, self.name)
+            if jmsg is None:
+                self.step_data.append((req_time + .1, step_data[0][1]))
+                return
+            last_time = jmsg['last_step_time']
+            if req_time <= last_time:
+                break
+        # Process block into (time, position) 2-tuples
+        first_time = step_time = jmsg['first_step_time']
+        first_clock = jmsg['first_clock']
+        step_clock = first_clock - jmsg['data'][0][0]
+        cdiff = jmsg['last_clock'] - first_clock
+        tdiff = last_time - first_time
+        inv_freq = 0.
+        if cdiff:
+            inv_freq = tdiff / cdiff
+        step_dist = jmsg['step_distance']
+        step_pos = jmsg['start_position']
+        for interval, raw_count, add in jmsg['data']:
+            qs_dist = step_dist
+            count = raw_count
+            if count < 0:
+                qs_dist = -qs_dist
+                count = -count
+            for i in range(count):
+                step_clock += interval
+                interval += add
+                step_time = first_time + (step_clock - first_clock) * inv_freq
+                step_pos += qs_dist
+                step_data.append((step_time, step_pos))
+LogHandlers["stepq"] = HandleStepQ
+
+
+######################################################################
+# List datasets
+######################################################################
+
+def list_datasets():
+    datasets = []
+    for lh in sorted(LogHandlers.keys()):
+        datasets += LogHandlers[lh].DataSets
+    return datasets
+
+
+######################################################################
+# Log reading
+######################################################################
+
+# Read, uncompress, and parse messages in a log built by data_logger.py
+class JsonLogReader:
+    def __init__(self, filename):
+        self.file = open(filename, "rb")
+        self.comp = zlib.decompressobj(31)
+        self.msgs = [b""]
+    def seek(self, pos):
+        self.file.seek(pos)
+        self.comp = zlib.decompressobj(-15)
+    def pull_msg(self):
+        msgs = self.msgs
+        while 1:
+            if len(msgs) > 1:
+                msg = msgs.pop(0)
+                try:
+                    json_msg = json.loads(msg)
+                except:
+                    logging.exception("Unable to parse line")
+                    continue
+                return json_msg
+            raw_data = self.file.read(8192)
+            if not raw_data:
+                return None
+            data = self.comp.decompress(raw_data)
+            parts = data.split(b'\x03')
+            parts[0] = msgs[0] + parts[0]
+            self.msgs = msgs = parts
+
+# Store messages in per-subscription queues until handlers are ready for them
+class JsonDispatcher:
+    def __init__(self, log_prefix):
+        self.names = {}
+        self.queues = {}
+        self.last_read_time = 0.
+        self.log_reader = JsonLogReader(log_prefix + ".json.gz")
+        self.is_eof = False
+    def check_end_of_data(self):
+        return self.is_eof and not any(self.queues.values())
+    def add_handler(self, name, subscription_id):
+        self.names[name] = q = []
+        self.queues.setdefault(subscription_id, []).append(q)
+    def pull_msg(self, req_time, name):
+        q = self.names[name]
+        while 1:
+            if q:
+                return q.pop(0)
+            if req_time + 1. < self.last_read_time:
+                return None
+            json_msg = self.log_reader.pull_msg()
+            if json_msg is None:
+                self.is_eof = True
+                return None
+            qid = json_msg.get('q')
+            if qid == 'status':
+                pt = json_msg.get('toolhead', {}).get('estimated_print_time')
+                if pt is not None:
+                    self.last_read_time = pt
+            for mq in self.queues.get(qid, []):
+                mq.append(json_msg['params'])
+
+# Main log access management
+class LogManager:
+    error = error
+    def __init__(self, log_prefix):
+        self.index_reader = JsonLogReader(log_prefix + ".index.gz")
+        self.jdispatch = JsonDispatcher(log_prefix)
+        self.initial_start_time = self.start_time = 0.
+        self.datasets = {}
+        self.initial_status = {}
+        self.log_subscriptions = {}
+        self.status = {}
+    def setup_index(self):
+        fmsg = self.index_reader.pull_msg()
+        self.initial_status = status = fmsg['status']
+        self.status = dict(status)
+        start_time = status['toolhead']['estimated_print_time']
+        self.initial_start_time = self.start_time = start_time
+        self.log_subscriptions = fmsg.get('subscriptions', {})
+    def get_initial_status(self):
+        return self.initial_status
+    def available_dataset_types(self):
+        return {name: None for name in LogHandlers}
+    def get_jdispatch(self):
+        return self.jdispatch
+    def seek_time(self, req_time):
+        self.start_time = req_start_time = self.initial_start_time + req_time
+        seek_time = max(self.initial_start_time, req_start_time - 1.)
+        file_position = 0
+        while 1:
+            fmsg = self.index_reader.pull_msg()
+            if fmsg is None:
+                break
+            th = fmsg['status']['toolhead']
+            ptime = max(th['estimated_print_time'], th.get('print_time', 0.))
+            if ptime > seek_time:
+                break
+            file_position = fmsg['file_position']
+        if file_position:
+            self.jdispatch.log_reader.seek(file_position)
+    def get_initial_start_time(self):
+        return self.initial_start_time
+    def get_start_time(self):
+        return self.start_time
+    def setup_dataset(self, name):
+        if name in self.datasets:
+            return self.datasets[name]
+        parts = name.split(':')
+        cls = LogHandlers.get(parts[0])
+        if cls is None:
+            raise error("Unknown dataset '%s'" % (parts[0],))
+        if len(parts) < cls.ParametersMin or len(parts) > cls.ParametersMax:
+            raise error("Invalid number of parameters for %s" % (parts[0],))
+        subscription_id = ":".join(parts[:cls.ParametersSubscriptionId])
+        if subscription_id not in self.log_subscriptions:
+            raise error("Dataset '%s' not in capture" % (subscription_id,))
+        self.datasets[name] = hdl = cls(self, name)
+        self.jdispatch.add_handler(name, subscription_id)
+        return hdl