What is data trapezoid form?
I got the answer from the professor after the assignment was graded during review. I am posting it here as it may help someone else in the future. Apparently, (vintage) data in trapezoid form for GDP are provided at Philadelphia Fed.
They have a trapezoid form because when they are plotted on a contour graph they have a trapezoid shape (the graph is from a correct review sample assignment from our professor):