Get a way to write an HTML legend
Make the legend informative
Make it possible to toggle legend visibility

This change adds an options menu with a "Show legend" checkbox.
Make legend work in landscape mode

With this change in place, the legend looks fine even when the device is
in landscape mode.

Previously the legend was only usable in portrait orientation.
Merge branch 'add-legend'
Make double tap toggle between two zoom levels

One is max zoomed out (as before) and one is zoomed in on the last two
days.
Animate double-tapping zooms
Fix an edge case crash

The edge case happens when you charge your phone short enough that both
the before and after samples says "draining", but long enough that the
after sample has higher charge percentage than the before one.

Closes #1