Include / document / improve single-colour chart options

[asongtoruin]

Particularly when it comes to the bar charts, I can see the merit in making a chart using a single colour for the bars (indeed, I can see the merit in that being the default behaviour). I can't quite work out whether this is possible at present.

I guess from the definition of get_colours() it should be, if you provided cmap as a list containing a single colour?

https://github.com/JackMcKew/pandas-alive/blob/ac770eecb2173e2d06fa852b0d65c03674f18b98/pandas_alive/charts.py#L110-L125

At a minimum it'd be nice to document this if it is possible, and maybe rework it to make it a little easier to just use one colour.

[JackMcKew]

This functionality is included in 6b62992

Use-case: df.plot_animated(cmap='b')

Example output:

https://github.com/JackMcKew/pandas-alive/blob/6b629924f0637d8c28d5323750a6c73571a76792/pandas_alive/charts.py#L110-L117

If a string is passed through the cmap keyword, and is not a defined colormap, this will then search the base colours of matplotlib and generate a repeating list of the RGB of this colour

Supported single colours:
https://matplotlib.org/2.0.2/api/colors_api.html