default browser status: x-axis label rounding error

[bcolloran]

on this view: https://moz-distribution-viewer.herokuapp.com/chart/13/

the x-axis labels are (0,1,1); should be [0, 0.5, 1] -- or actually ideally percentages [0%, 50%, 100%] to match the "This shows the default status of each FF client as a percentage of time; most clients have FF set as default either 0% or 100% of the time, but some clients switch default status."

[openjck]

Just as a note to self, this also happens in metrics-graphics:

metricsgraphics/metrics-graphics#707

[openjck]

Note that #221 will address many of the cases where this could happen but not all of them. It would be better to choose how many significant digits should be shown on a chart-by-chart basis. d3 has precisionRound and friends for this, but it'll take some thought to figure out how to best find the step.

Let's leave this open even after #221 lands and I can follow it up with an even more general fix.