Follow the generic plugin installation instructions in the FIR wiki.
The fir_stats plugin allows you to view statistics on incidents/events.
The plugin adds a "stats" menu on the top left of FIR, which contains graphs and other utilities to have a comprehensive view on past incidents/events recorded in FIR.
A GET parameter, ?date=<yyyy-mm-dd>, can be used on all pages to change the date range for which stats should be displayed.