Perhaps, nobody realized the question was related to Microsoft charts.
Nevertheless, the chart has a SaveImage method, that writes to a disc where you have specified. See below for a sample.
chartName.SaveImage(filePath, ChartImageFormat.Png);
You can then open the image using a Image stream, and then add a water mark text. Or do as shown in the sample below.
using (MemoryStream imageStream = new MemoryStream())
{
//Save the Image to a stream
chartName.SaveImage(imageStream, ChartImageFormat.Png);
//create a bitmap object.
Bitmap bitmap = bitmap.FromStream(imageStream);
Graphics myGraphics = Graphics.FromImage(bitmap);
//set the watermark text to be added as a watermark.
string watermarkText = "Place the watermark text";
Font watermarkFont = new Font(new FontFamily("Arial"), 18.0, FontStyle.Bold | FontStyle.Italic);
//get size of the string.
SizeF size = myGraphics.MeasureString(watermarkText, watermarkFont);
float X = bitmap.Width / 2 - size.Width / 2;
float Y = bitmap.Height / 2 - size.Height / 2;
//Draw the text
myGraphics.DrawString(watermarkText, watermarkFont, new SolidBrush(Color.DarkMagenta), X, Y);
//Save the new image with the watermark text
using (MemoryStream watermarkedStream = new MemoryStream())
{
bitmap.Save(watermarkedStream, ImageFormat.Png);
}
}
Hope this helps.