Make height of "Commits per author" graph flexible
This patch changes how the the "Commits per author" graph is generated. Right now this graph is always 400 pixels high. But if a project has many contributor, say 50, then the result is that for each author, only 8 pixels are available vertically. The results ranges from looking cramped to being totally unreadable.
The simple solution: Scale the graph based on the number of fields visible vertically.
This resolves issue #1983.