Unfortunately, our DashXL dashboards don't support graphs yet. We are already thinking about including them though.
The data logging will let you replay the data log in DashCommand, or export it and replay it in ScanXL (which does have graphs).
The default skin now included with the latest release of DashCommand also has an MPG bar graph. It looks like this:
Other than adding a line graph, our DashXL dashboards can do just about anything you can think of. You can look at DashXL.net to see some of what has already been done.