Workflow Tasks data in Pivot Chart

All, Has anybody ever tried to create a pivot chart in excel based upon the workflow tasks list in sharepoint? One of my users is requesting a chart showing All users associated with active or completed workflows AND they want to see the time between when it was assigned and when it was completed. I figured this would be easy, things i have tried - pulling list in view odata: success - creating chart from imported area: failure (workflow task list only has ID.) - pull in Userinformationlist and create a relationship between assigned ID and name in user informatiion list: failure (cannot merge named ranges from external data) - create calculated fields in workflow task list: failure (excel throws error importing a calculated field)

 I would use the history list, but this gets purged in our system every 60 days, and I cannot change. not trying to re-invent the wheel here, but the easiest way would seem to just create an event handler to write the values i want to another list on every change of the workflow history list, then pull that into my excel sheet later. I am not an expert with pivot tables and i have not been able to lookup the userid value against the UIC with any success.