jfreechart generates "one time images". should generate "per-session" images

In ChartHelper we call

ServletUtilities.saveChartAsPNG(chart, width, height, renderingInfo, null);

That "null" tells jfreechart to generate a "one-time" image. As soon as jfreechart has sent it out to someone it deletes it. This causes problems in IE6 which seems to have a bug where it sometimes requests the same image twice in a row. This results in the second request getting a 404 (since jfreechart already deleted it).

jfreechart also supports "per-session" images. We just need to pass in the HttpSession instead of null and jfreechart will keep the image around for the session.

Andreas thinks this change was supposed to have happened in 4.0.x but it appears to have fallen through the cracks. We should put it into 4.1.x