Add support for stacked with automatic calculation of the differences

[bobbingwide]

The stacked toggle option currently stacks the values on top of each other

eg Key,A,B,C X,1,2,3 Y,4,5,6

Would produce a chart for X going up to 6, and Y would go up to 15.

But sometimes we want to have the system display the stacked values by working out the difference automatically. So X would go to 3, and Y would go to 6.

What would the toggles be?

• stacked
• calculate differences