I need help figuring out how to plot cummulative data. I have product and
sales data in a table, broken down by quarter. For each product I want to use
a line graph to show cummulative sales over time.

For example, Product A has Q1 sales of $1M, Q2 of $1.5M, Q3 of $1.25M and Q4
of $1M. So the first point would be at 1M, #2 at 2.5M, #3 at 3.75 and # 4 at
4.75.

I can't find which format to use to make this kind of graph.