You say you talked to him about doing this project in January, but you didn’t and brought it back up in February. You’re saying you have been really busy. Could it be that he feels this project would take away from the pertinent work you are doing for him right now? Maybe it’s not that he doesn’t feel you’re competent to do it but feels that it is not as important for you to do as his other work?
If you’re thinking this cannot be the case then you can just go in and ask him. “I offered to do the charts but I am aware that you delegated it to “Sally. Do you have a concern with me performing this task?” or however you want to word it. Just make sure not to focus on this girl and sound insecure. Go in with confidence.
Hopefully he will be honest and you 2 can clear the air.
Let us know what happens!