Fix XY-Scatter Plots Colors - Rebased

[ada-x64]

This PR continues the work in #2258. It needed to be rebased and tweaked. @zxy994 your commits are preserved.

[texodus]

Thanks @zxy994 @ada-x64

For charts with only a split_by, the chart no longer colors scatter points individually (as discussed) - but it should use the default color theme color var(--d3fc-series) (which is a pale blue in the Pro theme), instead it seems to be picking the first distinct color from the distinct color series calculation:

[ada-x64]

@texodus Amended to use default colors. Also tweaked some code to be more readable.

[texodus]

Thanks for the PR @zxy994 @ada-x64! Looks good!

I think there are still a few areas this should be improved on in the future:

• We definitely need to improve the legend situation when both split_by and color columns are specified - right now it only shows them symbol legend (and I'd argue if we're to choose one, it should be the color legend).
• Mapping split_by to symbols isn't great to begin with - we should add a new column for symbol and make split_by a multi-chart ala "Treemap" and "Sunburst" charts.
• It's disorienting when switching from split_by to non-split_by configs with a color column, as the color palette change seeming arbitrarily (because the columns are named differently).
• In the future, we really want to see tests for all commits. As this PR has gone through 4 rounds of code review, we'll hold of on these but please aim to bring test coverage with all PRs.