In order to define a contextual action for cross-plot windows, you need to put your Python script in your Python directory under a directory called Contextual_Cross_Plot_Window.
Go to Tools->Options->Environment->Python Scripts (or Tools->Options->Python->Scripts in INTViewer 5.2) and click "Refresh from Disk". Your script should be listed automatically.
You will now be able to execute this script from the contextual menu of the cross-plot window:
This script will add a linear trend to all the cross-plot layers within the window.
This script is attached to the bottom of this article.
The result after the script executes should look something like the following.