Calculating a monthly backlog using raised and completed dates

[KBrown3456]

Hi

I am making a graph which is in the attached document.
I have a list of activities that show the activity code, date it was created and date it was closed.
I want the graph to show two lines one tracking amount of activities closed per month and one showing activities raised per month.
So far i have achieved this but now i also want to add the backlog for each month as a column graph behind the two line graphs.
I am struggling to calculate the backlog for each given month using the raw data i have to hand.

If anyone could help that would be great

[AliGW]

So what constitutes backlog?

[KBrown3456]

The backlog would be the count of how many activities are open per month

[JohnTopley]

... so is this not "Raised" - "Closed"?? (Or cumulative of the same)