Improve rendering and readability of labels

[abought]

The current plot label code is iterative on render. For a region with many labeled items, this triggers many forced refreshes, and causes the page to show a slow, ugly animation as labels move into position.

1. The layout code should be performed before labels are rendered, and explore more performant algorithms
2. Explore readability improvements for labels, lines, and backgrounds where items overlap with markers (like scatter plot points)

This will require a bit of experimentation and refactoring- priority is based on the effort:reward ratio. If specialized use cases arise where we want to show that many labels more often, we can revisit priority.