Codecov Report

❌ Patch coverage is 78.94737% with 48 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.80%. Comparing base (fd11c33) to head (5942e13).

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+ Hits         1526     1656     +130     
- Misses        293      320      +27     

Note: when alpha_fill = 1.0, you only see half of the outline, since the "inner part" lies behind the actual shape. So the outline will appear narrower than you might expect from the outline_width parameter

Regarding the "outline lies partly behind shape and will shine through if fill_alpha != 1" issue:

• a nicer solution might be to upscale the shapes that are used for plotting the outline(s) so that the outline perfectly lies around the shape
• for circles, that's easy: in the _process_point() sub-method in utils._get_collection_shape(), one can compute added_size_for_outline = 0.5 * linewidth and add that value to the radius of the mpatches.Circle objects
• however, for polygons it gets more complicated, since the points whould have to be shifted away from the centroid by a value that does not only depend on the linewidth but also the angle. I did not reach a satisfying solution there and it would get even more complex with concave shapes or multipolygons
• there might be an easier solution using shapely, but so far we think it's too much effort/perfectionism for too little improvement. So for now, we stick to the default matplotlib/datashader behavior.

Updates: we introduced a Color class that stores colors together with an alpha value.
Colors can be passed by the user as color name (e.g. "red"), an RGB(A) array (e.g. [0, 0, 0]) or a hex string ("#000000") in the format "#RRGGBB" or "#RRGGBBAA".
When alpha is set manually via a parameter, this value takes precedence (without being multiplied by potentially implicated alpha values from the color or a colormap). Else, if the hex string/RBGA array implies an alpha, this alters the opacity of the object. This behavior mirrors matplotlib. This is also true for outlines of shapes. When there are two outlines, they can therefore also have different opacity.
Change of shape outline plotting behavior: it would be sufficient to specify an outline color and/or width without setting the outline_alpha parameter in order for outlines to be drawn with alpha=1.0 (or whatever is specified by color)