You can do this using a graph instead of a cursor. You probably want to use a cursor to point to that position on the graph, and then you can use the function GetGraphCursorIndex, which obtains the plot handle and array index of the point to which the cursor is currently attached. That, along with a few other cursor attribute/value functions would probably be all you need to do this. If you need to stick with a chart, then you only have the x and y coordinates of the mouse and the size of the chart. You'll have to do some math to line up an x coordinate to an index value, and also maintain a growing array of points that are being plotted. Then, by clicking on the chart, you get the x coordinate, translate that to an index value, and then go to the global array and find out the Y value for that index. Hope this helps!
Jeremy L.
National Instruments