jschart does not restrict the average and median to the scope of the viewport

[RobertKrawitz]

If I restrict the horizontal view of a graph, I would like the average and median displayed in the accompanying chart to be restricted to the viewport rather than always being over the whole graph.

Use case: I'm measuring a process that has a startup phase and a steady state phase. I would like to restrict the graph to the steady state phase and compute the average and median for that restricted view.