My first Report - need a cumulative line?

Hi, I am building my first report but now need a Cumaltive count line adding.  Below is my simple Graph which shows a field "Income" shown across time.  Bars below the line are Costs and Bars above the line are income.  I need a cumalative line adding which adds the values up cumalatively to show my current £ balance.

I assume I create a new column which i can then add onto the graph.  What would be the formula for this new column?

Thanks in advance.

Steve