Use the default template's background color for swatches

[joelostblom]

This is a suggestion to use the default template's background color for swatches Instead of always using the plotly template. The potential benefit of this change would be that it is easier to tell which colors sequence go well with the currently used template.

It would be neat if this also checked if there was a PX template set, and I asked a related question about how PX and PIO templates synergize in #1869.

Screenshots:

[nicolaskruchten]

Looks good to me if you can fix the formatting problem in CI?

[joelostblom]

I believe this is the same CI error as on the other PRs and unrelated to the changes I have made. The error is

error: cannot format /home/circleci/project/packages/python/plotly/plotly/basedatatypes.py: cannot use --safe with this file; failed to parse source file.  AST error message: misplaced type annotation (<unknown>, line 256)

I think that line 256 might need to have collections. added in front of OrderedDict although I'm not sure since that is a comment.

[nicolaskruchten]

If you want to have this check the px.defaults.template and use that I'd be fine with it.

[joelostblom]

Thanks, added!

I also added the default width, so that the autoadjusting behavior can be overridden if desired.

width 600 samples:

[nicolaskruchten]

re the synergization of px and pio templates... Really the way to do this would be to rewrite this little swatch function on top of px.bar somehow :)

[nicolaskruchten]

the plot titles are also pretty ugly at the moment... they used to say px.colors.whatever but after to move into plotly.py, things got shuffled around

[joelostblom]

Rebased and cleaned up the titles

[emmanuelle]

Closing since #1886 was merged, thanks @joelostblom !