Ok, you are hovering over a VERY dark art, that is neither supported or documented.
Each object on a page is a "resource". You can add any amount of different types of resources to a view. It sounds like you'd need to edit the resource that contains the chart you want to modify, rather than the whole page.
That's where my knowledge of this type of modification ends, although it is something I've wished were possible for a long time.
Remember these changes are NOT supported - if you make a change and fry your install, you'll be guided to do a repair which MAY fix it. Also, when you upgrade or run the config wizard, it is likely your changes will be overwritten, so backup, backup, backup, and if you manage to make the changes you want, DOCUMENT how you did it, as one day you'll need to repeat it.
Another way this might be possible (if you have the SQL you need) is to write a custom report and put a "report from report writer" resource on the page instead.
I'd love to here how you get on.