Show individual points in line graphs when possible

Right now, we show the individual points on a line graph when during a hover:


We should add an option to line graphs to show _all_ points that make up a line, not just the currently hovered points, whenever there is space.

This will help users who end up plotting lines with only a single point, as well as users who want to see their actual data set as part of an interpolated line.

- [ ] Points should only show when for every line in the line layer, each point in the line is at least `2 * pointRadius` away from all other points in the line
- [ ] Update the points to have a "hollowed out appearance":

The hovered state of the point should put a dot in the center of the point.

No files or tests are named. Start by locating the line-graph rendering and hover-point behavior, then trace how line-graph options are configured. Done means an option can show hollow points when the spacing rule holds for every line, with a centered dot on hover, while avoiding overlap.

Written by the indexing model from the issue text.