add points-only demo, plus point density control

[leeoniya]

it's not terribly obvious how to achieve a series that only renders points. people have tried various things:

• setting width: 0 is not ideal because the path is still technically created (wastes CPU), and this prevents the series color from being displayed in the legend (which might be a bug) fixed.
• setting stroke: "transparent" has similar problems to above

the actual, non-obvious solution is to do paths: u => null.

it is now also possible to get fine-grained control of when the poins should be rendered (determined by avg x distance between points in the visible x range):

paths: u => null,
points: {space: 0},