If this information is somehow dependent on the horizonta/vertical axis values, this means that they are indeed data-dependent and I think it is a better choice to use a series to render this 'trend line'. Another option you have, but I wouldn't suggest you go with it, is to use the conversion api of the chart (ConvertDataToPoint method) and manually draw some polylines over the chart. These should be on top of the chart as separate UI elements and should not be part of the chart's visual tree. You will need to redraw them every time the size changes, the chart is zoomed and when the axes range change. I hope that you see that the second approach requires a bit more work, this is why I suggest you use a series for this purpose.
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items