解决方法
//在坐标(pt.X,pt.Y)上渲染带有基线的文本：
Font myFont = Label1.Font;
FontFamily ff = myFont.FontFamily;
float lineSpace = ff.GetLineSpacing(myFont.Style);
float ascent = ff.GetCellAscent(myFont.Style);
float baseline = myFont.GetHeight(ev.Graphics) * ascent / lineSpace;
PointF renderPt = new PointF(pt.X,pt.Y - baseline));
ev.Graphics.DrawString("Render this string",myFont,textBrush,renderPt);
