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g.FillRectangle(Brushes.Black, new Rectangle(0, 0, pnlGraph.Width, pnlGraph.Height));
Fill in the BackColor property of your panel in design mode. Maybe it's not much, but it helps if the background of your panel is already black, instead of having to repaint it every time from white to black.
Also the line pnlGraph.Invalidate(); near the end of the same handler is not needed.
I was also wondering why you have to change an integer value to a float value. This causes extra overhead, which, perhaps, is not needed.