fix coverage/index.html on mobile

[rdancer]

The squares have too much space between them vertically. The gap is just awful, takes 50% screen estate.

The legend could probably also be inline-box on mobile, to use the now-empty space to its right.

Single click on the box should summon the hover tooltip.

If the squares are 1px smaller, and if the gap is 0.5px on retina, they ought to fit on one screen.