According to JIRA Charting Plugin:
Time to First Response - displays the number of hours taken to respond to issues in a project or for a filter. Gadget only (i.e. not available as a report).
There should be an option for users to choose whether to display the time in hours or minutes. This will be helpful in case the time to first response is mostly less than 1 hour which is currently calculated as 0 hour, hence is not displayed.
JIRA 7.1.2, Charting 2.3.0