Remaining subplot problems

[etpinard]

A list of open items regarding the current implementation of subplots.

Bugs

• gl2d and cartesian subplots can't coexist. Fixed by @dfcreative 's regl-based scattergl

Improvement needed

• A better-looking and more scalable mode bar (see Subplot mode bars #258)
• A more consistent handling of orphan subplots across subplot types (see A slightly less ambitious take on orphan subplots and 'delete last trace' #289 and Sanitize orphan subplot logic and 'last trace' deletion #268 for more info)
• Sanitize subplot data join step in a way that consolidates svg and gl - e.g. no more selectAll().remove(). See https://gist.github.com/etpinard/744228c927ee95871559
• Ordering of subplot isn't ideal. Current order: cartesian / pie / ternary / gl / geo with no way to configure it.

[etpinard]

With @alexcjohnson 's #2227 and @dfcreative regl-based scattergl work, I think we can close this issue and its corresponding milestone.