add average ratio per day to bubble chart

in order to get a better idea of the average test code ratio, I am
adding a line into the bubble chart giving the average ratio. Because it
is unintuitive to indicate the average before the commits happen, I am
displaying the average for the day on the marker of the next day.

Also updated the Legend to update the line chart when any committers are
de/selected.
increase default amount of commits to display

the default number of commits retrieved when no query string is set is
50. This will not show too much on the chart then, so I am increasing it
to show 500.