Adjust y-scale of used/vetoed plot

[jrsmith02]

There is a problem with the y-scale on the used/vetoed plot. For example:
[https://ldas-jobs.ligo-la.caltech.edu/~laura.nuttall/hveto/20160922/1158549317_1158595217/plots/L1-HVETO_USED_FREQUENCY_TIME_ROUND_2-1158549317-45900.png](this plot)
The y-scale should extend up and down to encompass the highest/lowest SNR of the vetoed triggers and the used triggers.