TypeError when rendering a Holomap with Matplotlib whose data is a Dask DataFrame

[maximlt]

Rendering an histogram whose source is a grouped-by dask array raises an error (TypeError: len() of unsized object) with the Matplotlib backend (works fine with Bokeh and Plotly).

import holoviews as hv
from hvplot.sample_data import airline_flights
from holoviews.operation import histogram

hv.extension('matplotlib')

flights = airline_flights.to_dask().persist()
flight_subset = flights[flights.carrier.isin(['AA', 'US', 'OH'])]
by = 'carrier'

ds = hv.Dataset(flight_subset, by)
print(ds)

ds_grouped = ds.groupby(by)
print(ds_grouped)

hists = histogram(ds_grouped, dimension='depdelay')
print(hists)

hists

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/IPython/core/formatters.py in __call__(self, obj, include, exclude)
    968 
    969             if method is not None:
--> 970                 return method(include=include, exclude=exclude)
    971             return None
    972         else:

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/holoviews/core/dimension.py in _repr_mimebundle_(self, include, exclude)
   1314         combined and returned.
   1315         """
-> 1316         return Store.render(self)
   1317 
   1318 

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/holoviews/core/options.py in render(cls, obj)
   1403         data, metadata = {}, {}
   1404         for hook in hooks:
-> 1405             ret = hook(obj)
   1406             if ret is None:
   1407                 continue

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/holoviews/ipython/display_hooks.py in pprint_display(obj)
    280     if not ip.display_formatter.formatters['text/plain'].pprint:
    281         return None
--> 282     return display(obj, raw_output=True)
    283 
    284 

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/holoviews/ipython/display_hooks.py in display(obj, raw_output, **kwargs)
    256     elif isinstance(obj, (HoloMap, DynamicMap)):
    257         with option_state(obj):
--> 258             output = map_display(obj)
    259     elif isinstance(obj, Plot):
    260         output = render(obj)

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/holoviews/ipython/display_hooks.py in wrapped(element)
    144         try:
    145             max_frames = OutputSettings.options['max_frames']
--> 146             mimebundle = fn(element, max_frames=max_frames)
    147             if mimebundle is None:
    148                 return {}, {}

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/holoviews/ipython/display_hooks.py in map_display(vmap, max_frames)
    204         return None
    205 
--> 206     return render(vmap)
    207 
    208 

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/holoviews/ipython/display_hooks.py in render(obj, **kwargs)
     66         renderer = renderer.instance(fig='png')
     67 
---> 68     return renderer.components(obj, **kwargs)
     69 
     70 

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/holoviews/plotting/renderer.py in components(self, obj, fmt, comm, **kwargs)
    408                 doc = Document()
    409                 with config.set(embed=embed):
--> 410                     model = plot.layout._render_model(doc, comm)
    411                 if embed:
    412                     return render_model(model, comm)

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/panel/viewable.py in _render_model(self, doc, comm)
    460         if comm is None:
    461             comm = state._comm_manager.get_server_comm()
--> 462         model = self.get_root(doc, comm)
    463 
    464         if config.embed:

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/panel/viewable.py in get_root(self, doc, comm, preprocess)
    518         """
    519         doc = init_doc(doc)
--> 520         root = self._get_model(doc, comm=comm)
    521         if preprocess:
    522             self._preprocess(root)

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/panel/layout/base.py in _get_model(self, doc, root, parent, comm)
    120         if root is None:
    121             root = model
--> 122         objects = self._get_objects(model, [], doc, root, comm)
    123         props = dict(self._init_params(), objects=objects)
    124         model.update(**self._process_param_change(props))

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/panel/layout/base.py in _get_objects(self, model, old_objects, doc, root, comm)
    110             else:
    111                 try:
--> 112                     child = pane._get_model(doc, root, model, comm)
    113                 except RerenderError:
    114                     return self._get_objects(model, current_objects[:i], doc, root, comm)

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/panel/pane/holoviews.py in _get_model(self, doc, root, parent, comm)
    262         child_pane = self._panes.get(backend, Pane)(state, **kwargs)
    263         self._update_plot(plot, child_pane)
--> 264         model = child_pane._get_model(doc, root, parent, comm)
    265         if ref in self._plots:
    266             old_plot, old_pane = self._plots[ref]

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/panel/pane/plot.py in _get_model(self, doc, root, parent, comm)
    191     def _get_model(self, doc, root=None, parent=None, comm=None):
    192         if not self.interactive:
--> 193             return PNG._get_model(self, doc, root, parent, comm)
    194         self.object.set_dpi(self.dpi)
    195         manager = self._get_widget(self.object)

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/panel/pane/markup.py in _get_model(self, doc, root, parent, comm)
     39 
     40     def _get_model(self, doc, root=None, parent=None, comm=None):
---> 41         model = self._bokeh_model(**self._get_properties())
     42         if root is None:
     43             root = model

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/panel/pane/image.py in _get_properties(self)
    107         if self.object is None:
    108             return dict(p, text='<img></img>')
--> 109         data = self._data()
    110         if not isinstance(data, bytes):
    111             data = base64.b64decode(data)

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/panel/pane/plot.py in _data(self)
    234             bbox_inches = None
    235 
--> 236         self.object.canvas.print_figure(b, bbox_inches=bbox_inches)
    237         return b.getvalue()
    238 

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/backend_bases.py in print_figure(self, filename, dpi, facecolor, edgecolor, orientation, format, bbox_inches, pad_inches, bbox_extra_artists, backend, **kwargs)
   2259                         orientation=orientation,
   2260                         bbox_inches_restore=_bbox_inches_restore,
-> 2261                         **kwargs)
   2262             finally:
   2263                 if bbox_inches and restore_bbox:

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/backend_bases.py in wrapper(*args, **kwargs)
   1667             kwargs.pop(arg)
   1668 
-> 1669         return func(*args, **kwargs)
   1670 
   1671     return wrapper

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/backends/backend_agg.py in print_png(self, filename_or_obj, metadata, pil_kwargs, *args)
    506             *metadata*, including the default 'Software' key.
    507         """
--> 508         FigureCanvasAgg.draw(self)
    509         mpl.image.imsave(
    510             filename_or_obj, self.buffer_rgba(), format="png", origin="upper",

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/backends/backend_agg.py in draw(self)
    404              (self.toolbar._wait_cursor_for_draw_cm() if self.toolbar
    405               else nullcontext()):
--> 406             self.figure.draw(self.renderer)
    407             # A GUI class may be need to update a window using this draw, so
    408             # don't forget to call the superclass.

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/artist.py in draw_wrapper(artist, renderer, *args, **kwargs)
     72     @wraps(draw)
     73     def draw_wrapper(artist, renderer, *args, **kwargs):
---> 74         result = draw(artist, renderer, *args, **kwargs)
     75         if renderer._rasterizing:
     76             renderer.stop_rasterizing()

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/artist.py in draw_wrapper(artist, renderer, *args, **kwargs)
     49                 renderer.start_filter()
     50 
---> 51             return draw(artist, renderer, *args, **kwargs)
     52         finally:
     53             if artist.get_agg_filter() is not None:

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/figure.py in draw(self, renderer)
   2779             self.patch.draw(renderer)
   2780             mimage._draw_list_compositing_images(
-> 2781                 renderer, self, artists, self.suppressComposite)
   2782 
   2783             for sfig in self.subfigs:

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/image.py in _draw_list_compositing_images(renderer, parent, artists, suppress_composite)
    130     if not_composite or not has_images:
    131         for a in artists:
--> 132             a.draw(renderer)
    133     else:
    134         # Composite any adjacent images together

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/artist.py in draw_wrapper(artist, renderer, *args, **kwargs)
     49                 renderer.start_filter()
     50 
---> 51             return draw(artist, renderer, *args, **kwargs)
     52         finally:
     53             if artist.get_agg_filter() is not None:

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/_api/deprecation.py in wrapper(*inner_args, **inner_kwargs)
    429                          else deprecation_addendum,
    430                 **kwargs)
--> 431         return func(*inner_args, **inner_kwargs)
    432 
    433     return wrapper

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/axes/_base.py in draw(self, renderer, inframe)
   2919             renderer.stop_rasterizing()
   2920 
-> 2921         mimage._draw_list_compositing_images(renderer, self, artists)
   2922 
   2923         renderer.close_group('axes')

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/image.py in _draw_list_compositing_images(renderer, parent, artists, suppress_composite)
    130     if not_composite or not has_images:
    131         for a in artists:
--> 132             a.draw(renderer)
    133     else:
    134         # Composite any adjacent images together

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/artist.py in draw_wrapper(artist, renderer, *args, **kwargs)
     49                 renderer.start_filter()
     50 
---> 51             return draw(artist, renderer, *args, **kwargs)
     52         finally:
     53             if artist.get_agg_filter() is not None:

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/patches.py in draw(self, renderer)
    605                 self._bind_draw_path_function(renderer) as draw_path:
    606             path = self.get_path()
--> 607             transform = self.get_transform()
    608             tpath = transform.transform_path_non_affine(path)
    609             affine = transform.get_affine()

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/patches.py in get_transform(self)
    271     def get_transform(self):
    272         """Return the `~.transforms.Transform` applied to the `Patch`."""
--> 273         return self.get_patch_transform() + artist.Artist.get_transform(self)
    274 
    275     def get_data_transform(self):

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/patches.py in get_patch_transform(self)
    775         # important to call the accessor method and not directly access the
    776         # transformation member variable.
--> 777         bbox = self.get_bbox()
    778         return (transforms.BboxTransformTo(bbox)
    779                 + transforms.Affine2D().rotate_deg_around(

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/patches.py in get_bbox(self)
    854     def get_bbox(self):
    855         """Return the `.Bbox`."""
--> 856         x0, y0, x1, y1 = self._convert_units()
    857         return transforms.Bbox.from_extents(x0, y0, x1, y1)
    858 

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/patches.py in _convert_units(self)
    764     def _convert_units(self):
    765         """Convert bounds of the rectangle."""
--> 766         x0 = self.convert_xunits(self._x0)
    767         y0 = self.convert_yunits(self._y0)
    768         x1 = self.convert_xunits(self._x0 + self._width)

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/artist.py in convert_xunits(self, x)
    201         if ax is None or ax.xaxis is None:
    202             return x
--> 203         return ax.xaxis.convert_units(x)
    204 
    205     def convert_yunits(self, y):

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/axis.py in convert_units(self, x)
   1489     def convert_units(self, x):
   1490         # If x is natively supported by Matplotlib, doesn't need converting
-> 1491         if munits._is_natively_supported(x):
   1492             return x
   1493 

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/matplotlib/units.py in _is_natively_supported(x)
     64     if np.iterable(x):
     65         # Assume lists are homogeneous as other functions in unit system.
---> 66         for thisx in x:
     67             if thisx is ma.masked:
     68                 continue

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/dask/array/core.py in __iter__(self)
   1480 
   1481     def __iter__(self):
-> 1482         for i in range(len(self)):
   1483             yield self[i]
   1484 

~/miniconda3/envs/hvplot-dev37/lib/python3.7/site-packages/dask/array/core.py in __len__(self)
   1354     def __len__(self):
   1355         if not self.chunks:
-> 1356             raise TypeError("len() of unsized object")
   1357         return sum(self.chunks[0])
   1358 

TypeError: len() of unsized object