In bottom axis of burn-down chart, let's show year only once.

[kfogel]

Along the bottom axis of the burn-down chart, let's show the year only once (say at January) instead of repeating it for every month.

[kfogel]

Will discuss with @cecilia-donnelly before implementing.

[kfogel]

Turns out @cecilia-donnelly agrees about desirability, but it's just harder to implement than we thought.
I've started branch 3-conditionally-show-year to work on it. It's not urgent. In the meantime, I might change to two-digit years on master to at least reduce the visual noise along the X axis.