Add "Actions per visit" (Average value) sparkline and graph
Change "Total time spent by the visitors" into "Average visit length"
Graphs showing time should display it in "minutes" instead of seconds
These modifications should also apply to other similar pages:
It could also process and report 'Unique page views' which are a better metric than Page views. This can be obtained by summing the nb_uniq_visitors for each page. For non day periods, we can use sum_daily_nb_uniq_visitors.
There would therefore be a new stats and sparkline below the graph.
(In ) Fixes #1297