Tweaks for selecting y axes for plots, especially ones with small ranges.

The previous range selection heuristic tended to be too aggressive in expanding
the range of the y-axis when dealing with values close to 0. For example, if
all of the values ranged between 0.65 and 0.7 then it would set the y-range to
somewhere near [-0.1, 0.7], making it impossible to see changes over time. If
all the values [0, 1e-4] then it would select a range close to [-0.1, 1e-4].

This change adds logic to avoid expanding the range of the y-axis too much. It
also modifies the 'ignore outliers' logic to select a range that includes 90%
of the data, as opposed to the checked-in version that uses 80%.
Change: 151071239