insights: only allow integer y-axis tic marks.

Created by: Joelkw

Right now, all insights show are search result count numbers. These can only ever be nonnegative integers.

In the future, we might allow you to chart data types that are not integer-only, at which point we may need to revisit this or insight-type-specify our axis defaults. But for now, it's safe to just constrain all y-axis labels to be integers.

This doesn't mean we want to adjust the vertical spacing – in the below insight, we should just not show the non-integer values, but the graph's series' use of space can overall be the same.