Several bargraphs are configured to show lines, connecting the tops of the bars.
This implies that the values in between can be interpolated (this is is the meaning of these lines).
But there is no meaningfull interpolation in this application. If I have 20 bugs at priority 'high' and 10 at priority 'low', it does not mean that I have 15 bugs at priority 'medium', nor do I have 19 bugs at a priority slightly below 'high'. The line implies this, however.
The lines are pure eyecandy and they're actually incorrect. They're added to the graph at line 115-121 in graph_api.php and I think they should go.