SVG-based image comparison

[rolyp]

• [x] check use of altair_saver (locally) to save .png
  • [x] brew install chromedriver
  • [x] allow chromedriver to be opened
• [x] extend test framework (pushing each significant change to server)
  • [x] create and open .png file from chart
  • [x] try .svg instead of .png
  • [x] enforce functional dependency of image on Vega Lite (cf. non-deterministic labels in Plotly)
  • [x] compare with baseline .svg saved previously
  • [x] remove .new.svg files when they compare equal
  • [x] retain option to compare Vega Lite (JSON) output
  • [x] tolerate changes to Vega Lite if image identical
  • [x] but in that case, update Vega Lite baseline
• [x] tidy-up pass
• [x] document new dual-file approval protocol
• [x] see if running tests in terminal shows output
• [x] delete mysterious alt.renderers.enable('mimetype')
• [x] update git aliases
  • [x] git approve
  • [x] git reject
• [x] CD/CD setup

  • [x] install chromedriver into macos-10.15 GitHub runner

    JSON-based (Vega-Lite) image comparison is nice and human-readable but the JSON encoding has too much intensional information. For example, adding an unused data transformation to an Altair image generates a different Vega-Lite specification, even though the image output is unchanged. Similarly if you rename a variable in the data transformation pipeline.

It might be worth reverting to PNG-based comparisons (as we used for Backfillz) for a more extensional notion of equivalence.

[rolyp]

@edwardchalstrey1 I'm finding comparing outputs as .json (Vega Lite) files a bit suboptimal. When we change our code, and the Vega Lite code changes as a result, it's not clear when the resulting image actually changes too. I'm going to experiment with generating .png or .svg (the latter would be better as it's a text format, and so can be more easily diffed), but I'll probably also retain the ability to generate/compare Vega Lite. (Comparing images will be problematic for other reasons, e.g. the actual change will often be much more obscure than a Vega Lite change.)