volatility: show/hide groups/individuals interactively

[nbokulich]

Improvement Behavior
click to show/hide individuals/groups.

Current Behavior
currently, can click on the group legend to show single groups, but this only does one at a time.

Proposed Behavior
Would be very useful to, e.g., drop one or more groups to focus on specific groups for comparison.

Comment
same with individuals (spaghetti) but no such feature exists. Would be very helpful to hide all spaghetti but one, for example, to compare an individual's trajectory vs. the group mean.

[nbokulich]

#86 was just a partial fix — we can select one ind/group, but it would be nice if we could select > 1.

This might just be an issue in vega... but worth exploring when we get the time.