Not clear how to plot two curves in GUI

[jfkcooper]

For many applications it is important to be able to see more than one dataset, more than one model, or ideally, sets of data and models on the same plot. This may be currently possible, but I can't see hwo to do it, or find GUI documentation to see how to do it

[rozyczko]

It is now possible in the feature branch. This will soon be available in develop and subsequently released for testing.