Colours used: support continuous colour encoding

[edwardchalstrey1]

Done/dropped:

• [x] what should happen when color encoding is continuous -- plot "used colour" points as lines
• [x] check if bin argument to alt.Color is required
  • [x] uncheck "bin" in notebook :x: schema validation error (False is not one of [True])
  • [x] set bin=None in test case :x: None is not one of [True]
  • [x] if Scale provides bins, does Color need to provide bin too? :x: yes
• test case -- can't seem to make an alt.Color with a continuous encoding
• [x] schema validation error when unchecking bin in notebook

[rolyp]

Notes on the hot mess that is Altair/Vega-Lite colour binning/scales:

• Color.scale is the function that transforms data domain to visual domain; when the data domain is continuous, scale can be continuous or discretising
• Scale.bin can be provided for discretising scales
• what does it mean to provide a continuous scale if colour is binned -- does binning happen after scale?
• what happens if both Scale and Color provide bins?

[rolyp]

@edwardchalstrey1 Morning. I don't seem to be able to create a Color object any more that doesn't specify a bin -- I had that problem locally before, but now it seems to be the case in the notebook too. Yesterday I had to bump Python to 3.9.8 (because GitHub Actions stopped supporting 3.9.7), and I wasn't able to do this in way that didn't involve regenerating poetry.lock. I pinned jsonschema to avoid that problem from last week, but I guess lots of other things got updated, and that might have included the altair version.

Either way, I seem to be able to pick a continuous Scale, but not to disable binning, so I'm not (at the moment) able to reproduce the original problem (since the colour encoding is never really continuous).