Add code to fix y axis labels for small values.

Before this change, a y-axis value of 0.08 was displayed as "80m",
presumably indicating 80 milli-somethings.

After this change, it shows as 0.08 as expected. The specifics of
how to choose the cutoff are open to debate, but this fixes the
weird display for my use case :)
Merge pull request #96 from mreid-moz/small_yax_values

Add code to fix y axis labels for small values.