Chart.ChartAreas.AxisY.IsStartedFromZero = false;
see if that helps !
also read questions on forum
http://social.msdn.microsoft.com/Search/en-US/netframework?query=y%20axis&rn=Chart Controls for .NET Framework Forum&rq=meta:Search.MSForums.ForumID(1f1c3c1b-2ead-47c9-bd27-883e86ed067e) site:microsoft.com&beta=0&ac=8