mousedown in a graph
 

I am trying to let the user select a number of points in a scatter graph
using the mouse but I am having difficulty converting the mouse down and
mouse up x y coordinates into x y values in the graph plot area which i was
going to use to select the items that fell within the range. There is also
the problem with draging the plot area around instead of selecting an area.
I would be most grateful for any assistance.