With that information, and re-reading your initial request, I realize that you’re representing projects as tasks (right?)? And those tasks are multi-homed to the “big” project-of-projects and to the actual project, if I understand. Dashboards are not set up for that–they’re not able to disambiguate that one is a “real” project and the other is the “collector” project.
If you’re on an Enterprise plan you could likely use a script action to maintain that project custom field so you don’t have to do it manually.
But the underlying issue is: If you put those projects in a portfolio (Advanced and above plans) then you’d be able to have project as an X-axis choice.
PS This won’t help, I don’t think, but I actually had a similar need in Asana2Go (Disclaimer: I’m the creator) so I’ve seen that issue, but I was able to address it in the scripting language I used to implement: