To get the position of mouse in you data, you should use something like this:
var transform = plotter.Transform;
var mouseScreenPosition = Mouse.GetPosition(plotter.CentralGrid);
var mousePositionInData = mouseScreenPosition.ScreenToViewport(transform);
and then you should somehow implement searching of proper y value by x value found by method I've described in your data.
Also you might be interested in AxisControlSamples in DevSamples.