Hello!
i see. this looks to be by design, although with the inconsistency between the two chart types, it might be considered a bug. you can submit a bug report to infragistics developer support (http://infragistics.com/gethelp) if you want, and the developer support engineer can give this issue the due investigation.
whatever the case, you can work around the problem by handling the chart's FillSceneGraph event, then finding the Polyline primitives and re-mapping the points which match the Y axis's MapMaximum. you can reference the axis using e.Grid["Y"] in the EventArgs, and the Polylines will be found in e.SceneGraph.
What I found out is that this only happens with StepLineCharts. I attached an image with 4 parts where you can see the different behaviour. Pic 1 and 2 show how an Line chart get cut off and Pic 3 and 4 show a StepLineChart that does not get cut off. Here a line is shown right at the boundary. (The left pics are with auto axis and the right pics are with intentionally to small axes.) To me this looks like a bug?
(By the way my IE (7) crashs all the time when I try to upload a file.)
Uli
it sounds like the clipping bounds are not right for the chart layer, but i'm not sure how to reproduce this problem. could you provide an image or sample code? there is an "options" tab visible when composing a message here, you can use that to upload an image of your chart or a sample project.