[Design] Add icon to sidebar histograms on right

[Makosak]

Issue
User can't easily understand what they are filtering

Recommendation
Add information icon next to variables on histograms sidebar, linking to metadata on that variable in a pop-up window (or perhaps window shows up on hover)

Rationale
Filtering the existing map, one variable is defined on left, but the filter (on right) is not clear -- won't know exactly what you're filtering unless you'd hunt for the variable on left user selection, if it's even there.

Additional context