How to create a cumulative chart of bugs trend over time based on priority?

[Maria Boyko]

There is a need for the next chart:

1. all bugs (can be done by filters)
2. cumulative
3. grouped daily/weekly
4. split by priority
5. over time (last N days)

In other words we need to see how many Blocker, Critical, Major bugs were there during last 30 days.