You can use the StringFormatFlags.DirectionVertical in the DrawString command to rotate text. Unfortunately, when drawing the graph it draws the text mirror-imaged the wrong way. So much for StringFormat.
In order to rotate text 90 degrees, you'll need to draw the text to a separate bitmap in memory and then draw the resulting image to the screen.
In your paint event handler code add the following:
SizeF labelSize = g.MeasureString(myLabel, GraphFont);
Bitmap stringmap = new Bitmap((int)labelSize.Height + 1, (int)labelSize.Width + 1);
Graphics gbitmap = Graphics.FromImage(stringmap);
gbitmap.SmoothingMode = SmoothingMode.AntiAlias;
// g.DrawString(m_LabelY, GraphFont, Brushes.Blue, new PointF(ClientRectangle.Left , ClientRectangle.Top + 100), theFormat);
gbitmap.TranslateTransform(0, labelSize.Width);
gbitmap.RotateTransform(-90);
gbitmap.DrawString(myLabel, GraphFont, Brushes.Blue, new PointF(0 , 0), new StringFormat());
g.DrawImage(stringmap, (float)ClientRectangle.Left, (float)ClientRectangle.Top + 100);
And don't forget to dispose of your bitmap and graphics objects at the end of onpaint
gbitmap.Dispose();
stringmap.Dispose();