You absolutely can combine statistics from 2 separate data sources into one graph.
You do this by creating multiple series within the component and have each series come from a different data source / view.
I am attaching a small video showing how to combine Incidents & Changes on the same time chart (my demo data didn't have any Request records). The key here is to make sure that all series are using the same time scale/interval. You will see in the video that the date field is cleared when I change the datasource/view of the series, so I have to go back in and reset it to a similar appropriate date range for the new series.