I used the following code (replace GetActiveZgc() with your ZedGraphControl object).
void ZoomReset()
{
GetActiveZgc().IsEnableHPan = false;
GetActiveZgc().IsEnableHZoom = false;
for (int i = GetActiveZgc().GraphPane.ZoomStack.Count * 2; i >= 0; i--)
{
GetActiveZgc().ZoomOut(GetActiveZgc().GraphPane);
}
GetActiveZgc().GraphPane.XAxis.Scale.Min = 0;
GetActiveZgc().GraphPane.YAxis.Scale.Min = 0;
GetActiveZgc().IsEnableHPan = true;
GetActiveZgc().IsEnableHZoom = true;
}