code: public void setBackgroundColour(Color backgroundColour) {
      if (backgroundColour == null) {
        backgroundColour = Color.WHITE;
      }
  
      this.backgroundColour = backgroundColour;
    }
docs: Sets the colour to be used on the background of the chart. A transparent background can be set
  by setting a background colour with an alpha value. The transparency will only be effective
  when the image is saved as a png or gif.
  
  <p>
  Defaults to <code>Color.WHITE</code>.
  
  @param backgroundColour the new colour to be set as the background fill.