Displays a y axis max value based on the data displayed in the chart

Closes #1821.

The problem is that the y max value for the y axis is based on the max value of all the data returned in an elasticsearch response. Elasticsearch may return buckets that fall outside of the time range specified (for more see my comments in #1787). In Kibana, however, we respect the time range the user selects and will only show data within that time range. Therefore, if the bucket that falls outside of the time range has the highest y value, then the y axis will use that value to set the max y axis value rather than taking the max value of the buckets displayed in the chart.

This pull request fixes this issue by pulling the y max value from the buckets displayed within the chart.