Break-even and Profit volume charts

Hi there,
I am having difficulties understanding how to draw up charts. I don’t seem to be able to understand how the units are being calculated so that the two lines (Total costs and Total revenues) cross in the appropriate point. And also don’t understand how you calculate the cumulative units. costs, revenues for Break-even charts – most profitable product sold first and for Profit Volume Charts for Multi Product Organisation.

Please help. I’m really lost here..

Thanks
Atty