http://blog.csdn.net/gisfarmer/article/details/3736452

Chart饼状图，每根柱子的宽度：

int a = Chart1.Series["Series1"].Points.Count;
  Chart1.Series["Series1"].CustomProperties = "PointWidth=0.5";
  Chart1.Width = Unit.Pixel(25 * a);

如果有多个Series，还可以在乘以Series的个数。