Sometimes it is helpful if the RendererState object gets notified if a plot pass is started and finished.
Example: A Renderer combines multiple values to one value (e.g. a mean of 5 values) and stores the intermediate results in the RendererState object. In case the chart is zoomed only a subset of dataitems are passed to the drawItem method but the drawItem method does not know how many items this will be. Therefore it is not possible to finish the last point of the dataset.
The attached patch gives a proposal how to change the RendererState class and the XYPlot class to fix this.
See also thread http://www.jfree.org/phpBB2/viewtopic.php?t=22535