Colormaps and colorbars with single color and varying alpha

[ianthomas23]

I would like to support colormaps/colorbars which use a single color but varying alpha. There are 2 use cases:

1. Datashader uses this approach for categorical data with a single color per category and an alpha that depends on the amount of that category or the total of all categories.
2. Contouring of cloud cover and pollution densities on top of maps, so the map is partially visible through the cloud/pollution.

As e.g. fill_color and fill_alpha are separate properties it is not sensible to add this functionality to the existing ColorMapper classes. So I propose to add new ScalarMapper classes which do pretty much the same as the ColorMapper classes but they map from data space to a scalar value rather than to a color. They could be used for other things than alpha, hence the more generic name. We would need separate LinearScalarMapper, LogScalarMapper and EqHistScalarMapper and from the user's point of view they could be hooked up to any scalar visual property just like the ColorMappers are hooked up to colors.

ColorMapper takes a sequence of colors. ScalarMapper could take a sequence of scalars but I think it more sensible for it to take start and end values (names TBD) to interpolate between. Probably it would have to accept an integer number of steps to be analogous to the number of palette entries for ColorMapper, but this could be optional if you wanted full-precision interpolation rather than the binned interpolation of ColorMapper.

Adding this to ColorBar isn't quite as clear. It currently takes a ColorMapper so we'd have to add an optional ScalarMapper for the alpha and either support a ColorMapper of a single value (this might work already?) or allow a single color to be specified as an alternative to the ColorMapper. Given that ColorBar returns a sequence of RGBA colors, the display of the colorbar shouldn't be a problem although we'd need to cope with different numbers of entries in the ColorMapper and ScalarMapper.

[mattpap]

As e.g. fill_color and fill_alpha are separate properties (...)

Just to note, fill_color can contain alpha channel now (since we allowed support for CSS4 colors).

[ianthomas23]

Thanks, I didn't know that. Which means I can accomplish everything I want using the existing colormapper and colorbar classes.