There are a couple of different ways of doing this.
Option 1 is to use the PointPair's Tag property when setting up your data. If the Tag is a string, it will be displayed as a tooltip for the point.
PointPair pp = new PointPair(....);
pp.Tag = "This is a custom tooltip";
Option 2 is to subscribe to the graph control's PointValueEvent and provide a custom value in your event handler.
graph.PointValueEvent += OnPointValueRequested;
...
private string OnPointValueRequested(object sender, GraphPane pane, CurveItem curve, int pointIndex)
{
PointPair point= curve[pointIndex];
string tooltip = String.Format("({0}, {1})", point.X point.Y);
return tooltip;
}
Also keep in mind that there is a bug with tooltip CPU usage on Vista and above. You may need to patch your copy of ZedGraph to fix it if you haven't already done so.