I figured out how create a diverging asymmetric colorbar in bokeh and use it in a bokeh chart. Was so happy about that I wrote a blog post about it! https://www.rbtechblog.com/blog/stocks2018#colorbar
And @anon10586594 has tweeted about it!
I am wondering if this is something you would like to put in the bokeh library. This is somewhere I could contribute to bokeh with a little guidance. First though, is it needed? I have used diverging asymmetric colorbars just a handful times in last few years as a data scientist that creates many visualizations. Too many high level functions can really clutter up the interface!
@russellburdt I think it could make a nice addition, but I would suggest some tweaks:
apart from figure, it’s atypical in Bokeh to have functions that return configured Models or assemblages of Models. So there isn’t an existing place this naturally fits in as-is. Instead I’d suggest a function that skips the last step, and returns only the array of colors.
the function can also generate symmetric diverging colormaps as a special case, so I’d suggest making the name more general
Taken together, I would propose adding a new function diverging_palette (or maybe generate_diverging_palette if we want to be more verbose). Such a function would make perfect sense to put inside the bokeh.palettes module. What do you think?
@Bryan Thanks for pointing me to the palettes module.
I see the contribution as resembling the linear_palette method, which takes palette and n as arguments. Towards that end a good name for the contribution would be diverging_palette (as you recommended) with arguments palette1, palette2, n, midpoint=0.5, and reverse=True. The function would return a list of hex colors representing the diverging palette (symmetric by default). An asymmetric diverging palette could be created by adjusting the midpoint argument away from its default value.
@russellburdt That sounds perfect. I’d suggests modeling tests and docs off the ones that exist already for linear_palette. Please let me know if you have any questions about those!
@Bryan Could you point me to information on tests and docs for this contribution.
Examples of palettes returned by diverging_palette are below. The function builds and returns a diverging palette from arguments palette1 and palette2. Optional arguments support a number of colors n and a relative location where the palette diverges midpoint. Default behavior is to return a 256-color symmetric palette.
@Bryan What do you think about adding additional 256-color palettes to bokeh?
In order to create the picture in my last response I needed to create a bokeh Reds256 palette and the same for Purples and Blues. There was already a Greys256 palette in bokeh 1.3.4.
The additional palettes can be created by hard-coding a list of hex colors created by matplotlib: palette = [matplotlib.colors.to_hex(x) for x in matplotlib.cm.Reds(np.arange(256) / 255)] palettes.Reds256 = palette
That is, the list would be hard-coded as palettes.Reds256 and matplotlib would not become a dependency of bokeh.
@russellburdt I am pretty indifferent about adding either new 256 color palettes, or new palette generating functions. I would suggest raising the issue in a PR so we can try to get other comments from @bokeh/dev.
For docs, I would just say imitate the similar analogous functions already in bokeh.palettes.
It turns out there is not much test coverage for palettes:
That’s probably because most of the content is just data. But I think though since your function computes new palettes it make sense to add a unit test or two in that test module to make sure that the expected output is generated in some test cases.
