We don't support comparing or combining data from multiple sources. This is a popular feature request, and it's on our product roadmap. Do let us know how you would use this feature.
Use groups to highlight relationships between metrics
The best option to compare data from different sources would be to display them side by side on the dashboard.
Our dashboards include a grouping functionality for your widgets to make the relationships between metrics on your dashboard obvious immediately. Just drag and drop one widget onto another to create a group.