Unexpected redraw in MFC dialog box

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MFC

When I draw a graph (put points) in the dialog box, about 70-80 percent is drawn, then redrawing starts from the very beginning. Then, at some point, the dialog box goes to the background, and after a while (after several redraws), the drawing ends and the dialog box comes to the foreground.

I thought that the redraw might be due to the fact that the dialog box is modal and the main application thread is blocked, and as a result, the drawing starts again. But I tried the modeless dialog as well. The result is the same

It should be noted that this behavior is observed only with a large number of drawn points. When there are few points, there are no redraws.

can anyone suggest why this behavior is observed

Thank u in advance!

What I have tried:

void CDialogGeometry::OnPaint()
{
	CPaintDC dc(this);

	CRect rect;
	GetClientRect(&rect);

	// here I calculate x and y of a point in a loop 
	//for (...)
	{
		//then I set a pixel
		dc.SetPixel(x, y, RGB(0, 0, 0));
	}
}