R1:02981f0add6d

Fix negative chart values, add storage for charts

Summary:
Ref T13279. I think I'm going to fling some stuff at the wall for a bit here and hope most of it sticks, so this series of changes may not be terribly cohesive or focused. Here:

The range of the chart is locked to "[0, 105% of max]". This is trying to make a pleasing extra margin above the maximum value, but currently just breaks charts with negative values. Later:

- I'll probably let users customize this.
- We should likely select 0 as…