Custom Reports are available for vSphere UI only.
There are a couple of sample xml files in the UI VM that you can take a look. I don't have access to VCOps VM right now but try to search for CustomViewsAndReports.xml in the UI VM.
The idea is that you define in those XML-s views (select view type - trend, list, distribution, select some metrics - here is the tricky part since you can use by default only the capacity related once, for the rest you need to do additional steps) and then include those views in the reports.
If you are still interested I can dig for you more information tomorrow.
Thanks,
Alex D.