One way i can think of doing this is to create an hourly rate adjustment tag which you would set to ×(multiply) and the value you require(1.5) then tag a work unit with a task set up with your standard hourly rate.
Then on the analytics page select: Tasks, Earnings and either bar or line. Set the period to the relevant month, you should now have the earnings including the overtime of that month displayed in the graph below.
