Support plots that use by with rasterize with hv.ImageStack (#1132)

* WIP overlaying rasterized

* Clean up

* Add rasterize to check for categorical

* Add test

* Add version check

* Update hvplot/converter.py

Co-authored-by: Simon Høxbro Hansen <simon.hansen@me.com>

---------

Co-authored-by: Simon Høxbro Hansen <simon.hansen@me.com>

hvplot/converter.py

@@ -582,7 +582,7 @@ def __init__(
                     symmetric = self._process_symmetric(symmetric, clim, check_symmetric_max)
                 if self._style_opts.get('cmap') is None:
                     # Default to categorical camp if we detect categorical shading
-                    if (self.datashade and (self.aggregator is None or 'count_cat' in str(self.aggregator)) and
+                    if ((self.datashade or self.rasterize) and (self.aggregator is None or 'count_cat' in str(self.aggregator)) and
                         ((self.by and not self.subplots) or
                          (isinstance(self.y, list) or (self.y is None and len(set(self.variables) - set(self.indexes)) > 1)))):
                         self._style_opts['cmap'] = self._default_cmaps['categorical']
@@ -1330,7 +1330,10 @@ def method_wrapper(ds, x, y):
                 opts['rescale_discrete_levels'] = self._plot_opts['rescale_discrete_levels']
         else:
             operation = rasterize
-            eltype = 'Image'
+            if Version(hv.__version__) < Version('1.18.0a1'):
+                eltype = 'Image'
+            else:
+                eltype = 'ImageStack' if self.by else 'Image'
             if 'cmap' in self._style_opts:
                 style['cmap'] = self._style_opts['cmap']
             if self._dim_ranges.get('c', (None, None)) != (None, None):

hvplot/tests/testoperations.py

@@ -3,12 +3,13 @@
 from unittest import SkipTest
 from parameterized import parameterized
 
+import colorcet as cc
 import hvplot.pandas  # noqa
 import numpy as np
 import pandas as pd
 
 from holoviews import Store
-from holoviews.element import Image, QuadMesh
+from holoviews.element import Image, QuadMesh, ImageStack
 from holoviews.element.comparison import ComparisonTestCase
 from hvplot.converter import HoloViewsConverter
 
@@ -194,6 +195,11 @@ def test_datashade_rescale_discrete_levels_default_True(self):
         actual = plot.callback.inputs[0].callback.operation.p['rescale_discrete_levels']
         assert actual is expected
 
+    def test_rasterize_by(self):
+        expected = 'category'
+        plot = self.df.hvplot(x='x', y='y', by=expected, rasterize=True, dynamic=False)
+        assert isinstance(plot, ImageStack)
+        assert plot.opts["cmap"] == cc.palette['glasbey_category10']
 
 class TestChart2D(ComparisonTestCase):