System.Drawing Problem in c#

Question

Hey there, Recently I've been writing a small app that draws graphs of any function and came across a very weird problem. After I initialize graphics using: Graphics _graph = this.CreateGraphics() And draw all my points using: _graph.DrawLine() I see the graph perfectly and everything works fine. But, then I added an event that makes a textbox visible anytime the user holds his mouse over one of the function's points. As soon as I hold my mouse over my func's graph, The graph disappears!!! And it happens only at the first time, if I draw the graph again using a button (that's on the form), It won't disappear again!!! Can anyone explain it to me!? I never ran into such a weird problem before... THANKS AHEAD! And a good day, Daniel.