Bring back dimension margins

We removed margins from dimension calculations since we didn't need them, and were trying to using them for the wrong thing at the time (scaling the pie chart widget's chart).

We kind of need them again:
  - line graph dots for the start and end points are only half-visible
  - #135 displays a hovered over bar's value on top of the bar, but these are cut off for bars that take up the entire chart width.

The plan is to add them back, but use them properly.