Suggestion: use altair for tree

[swo]

An alternative/complement to #55 : Rather than matplotlib, use altair

Nice things:

• You get things like the x axis for free
• You don't need to think about exactly where/how to draw things
• I find this way easier to reason/think about, but that's probably because I've been living in altair land

Disadvantages:

• Less future flexibility? Are there things we'd want to show that aren't in this plot?

[swo]

Closing in favor of matplot lib. We're pushing the boundaries of what altair can do easily, and the idea of full customization is nice.