/**
  * Sets the y label of the axes. The font names understood are those understood by
  * java.awt.Font.decode(). If the font name is null, the font remains unchanged.
  *
  * @param s the title
  * @param font_name an optional font name
  */
 public void setYLabel(String s, String font_name) {
   yaxis.setTitle(s, font_name);
 }