Unfortunately we have not designed the RadHeatMap to support such a scenario. This is also the first inquiry for such an output. Can you share with us some details about your current app, what is the data you are visualizing, and what meaning does this line have? What is the items source of this line, is it a result of the items source of the heat map, or is it fed by a separate data source?
If we find the described scenario important or common, we might add such a functionality with some future release. Currently this is not supported. We apologize for this inconvenience.
the Telerik team