How to show 2 date fields on the same line chart?

How to show 2 date fields on the same line chart?

I have a projects table. The table has 2 date fields. One for when the task is started and one for when the task is completed.

I want to show the total for each date field per year month.

It works if I put one of them in the X-axis but I obviously can not put them both in the X-axis.