I think you mean the axis scale property?
Try the following:
zedGraphControl.GraphPane.YAxis.Scale.MaxAuto = true;
Remember to call AxisChange() to force ZedGraph to recalculate the automatic scale:
zedGraphControl.AxisChange();
Finally, depending on how you redraw the chart, you may need to:
zedGraphControl.GraphPane.Invalidate();
source share