Pivot chart with subform data

Pivot chart with subform data

I need to create a chart with data from a form and a subform inside it.
But when creating report, all fields but subform fields appear in the list to be used. What should I do ?

Basically the subform has a list of "procedures".
I need a chart where X is the "date of procedure" contained in form

It should be easy, but I cannot find a solution.

Thanks for any help.