Draw barplots on their own "layer" of the canvas

Came up in discussion with @gwarmstrong today. I imagine this would work similarly to the plans for #452. This would make the process of rendering barplot updates much faster, and will be really useful when barplots are refactored (we could conceivably do things like updating individual barplot layers in isolation?)

Read the discussion around issue #452 first, since this issue says the proposed canvas-layer approach should follow those plans. Then locate the barplot rendering entry points and define how updates would be isolated to their own canvas layer; done means barplot updates render independently and measurably faster.

Written by the indexing model from the issue text.