j'ai fait comme ceci:

private static void drawVerticalString(string text, Point p, Font font, Form myForm)
        {
            System.Drawing.Drawing2D.GraphicsPath gp = new System.Drawing.Drawing2D.GraphicsPath();
            gp.AddString(text, font.FontFamily, (int)font.Style, font.Size+4, p, StringFormat.GenericTypographic);
            System.Drawing.Drawing2D.Matrix m = new System.Drawing.Drawing2D.Matrix();
            m.RotateAt(270, p, System.Drawing.Drawing2D.MatrixOrder.Append);
            gp.Transform(m);
            myForm.CreateGraphics().FillPath(new SolidBrush(Color.Black), gp);
        }