You need to use StringFormat to draw your string in vertical, using the flag. Try the following in your WndProc
method:
protected override void WndProc(ref Message m)
{
base.WndProc(ref m);
if (m.Msg == 0x000F)
{
using (Graphics graphics = CreateGraphics())
using (SolidBrush brush = new SolidBrush(ForeColor))
{
StringFormat format = new StringFormat(
StringFormatFlags.DirectionVertical);
SizeF textSize = graphics.MeasureString(Text, Font);
graphics.DrawString(
Text, Font, brush,
(Width / 2 - textSize.Height / 2),
(Height / 2 - textSize.Width / 2),
format);
}
}
}