How-to-place-a-horizontal-line-on-an-indicator-pane

For RSI, I know that the range is 0 to 100.

I have this code:

CODE:
   rsi = new RSI(bars.Close, 20);
         PlotIndicator(rsi, new WLColor(255, 0,0));
         _startIndexList.Add(rsi);

How can I place a horizontal line at a value of 20, 50 and 100 on this pane.

Thank you,
Larry