Fix rounding clobbering large values.

Previous behavior always rounded to 2 signifigant figures, clobbering
the difference between large values with small differences and making
their tooltips all display the same. This makes all values display to 2
decimals of precision; in the future, we may want to look at a more
dynamic option as a middle ground of fixed precision in all charts.

Author: Dylan Nugent
Fix y-axis label formatting in time-series charts

Time series charts previously always used 2 significant figures for
ticks on the y-axis, even if tick spacing was more precise than 2
significant figures, leading to charts that had repeated values along
their axes. Replace the lazy formatter with a smart formatting
function that uses the SI prefix for the largest Y value and displays
ticks to proper granularity.

Author: Dylan Nugent