more Discrete Scale documentation

[Mattriks]

• I've updated the documentation
• I've built the docs and confirmed these changes don't cause new errors

This PR:

• adds documentation pertaining to making scatter plots with different shaped markers (issues) #1366
• the main remaining point in making scatter plots with different shaped markers (issues) #1366 is "issue 3" which I leave to a future PR (size revamp).

[Mattriks]

Gadfly's Compose compat obviously needs to be bumped,. Would it also make things easier if common packages (e.g. Colors etc) have the same compats?

[bjarthur]

herein lies the problem with the polling (instead of interrupt style) mechanism used by CompatHelper. ideally the tagging of Compose should have triggered an immediate notification to Gadfly that it's compat section is out of date. but Github permissions don't allow actions to post PRs to other repos. so all CompatHelper can do is poll periodically. Gadfly and Compose are currently set to do so midnight on sunday, or whenever an issue is opened or re-opened.

[bjarthur]

just add another commit which updates Compose in compat.

[tlnagy]

Gadfly and Compose are currently set to do so midnight on sunday, or whenever an issue is opened or re-opened.

Why can't we just poll daily? It would be nice to batch the updates more than that, but it's not really a hassle.

[bjarthur]

we could, i just figured it's a load on the servers that isn't warranted, despite being free.

[codecov-io]

Codecov Report

Merging #1370 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1370   +/-   ##
=======================================
  Coverage   90.55%   90.55%           
=======================================
  Files          38       38           
  Lines        3961     3961           
=======================================
  Hits         3587     3587           
  Misses        374      374

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[Mattriks]

Rebased and ready to merge.