Summary page, and graphs, seems to fetch all bugs in the project.

This is not practical when the project has a lot of historic issues
A common use case is the need to have a summary of a subset of the issues, for a date interval.
Otherwise, the graphs get cluttered with old data and hides the relevant information for the recent activity.

A filter by date is needed to improve this situation.

I suggest reusing the filter system.
See my implementation for timeline search as an example of joining with a filter-generated query.
https://github.com/mantisbt/mantisbt/blob/master/core/history_api.php#L189

If done so, a further step into offering a complete filter for summary would be much easier to implement.
Note that my work on filter api will eventually provide the ability to paint and use a filter box in the summary pages.