Is it possible to use a formula in a chart?

 

I am wanting a graph of monthly costs.

 

I have two tables Product_cost , Product_extra, these two tables are joined on product_id.

 

In my report I group on product_id with a sum to add up product_extra, in the group footer I add sum of product_extra to product_cost   I would like this total to be in my graph?

 

Have tried creating a seperate formula that just shows the total of the above but still I cannot add this to my graph?