Ok, we're back at the starting point. As I wrote earlier, I found no simple solution.

The Exceltrend is more or less the same COM object as in ProcBook.
There are some ways to solve this:
-You use excel charts for trending (there is more functionality, but then you will lose all benefits of the PITrend )
-You develop your own Trend object (.NET of course).
-You extend the functionality of the existing COM trend object.

or you use the method you describe in the 1st post (set up a trace for this vertical line, this time as excel data)

All you need to do is change a couple of lines in the example code I posted.
Rather than adding the object, set the variable to the OleObject that represts the Trend and the ExcelData container. Then on the trend, clear any existing datasourceenvironments, configure your ExcelData using AddQuerySpecs method then add the ExcelData as a DataSoruceEnvironment for the Trend.

Should find a few minutes spare in the morning to post the code for what I mean.

Good idea, Rhys