RSI plotting custom array

Hi.. am trying to create a ratio chart.. I also want to plot RSI based on the ratio chart.. I tried using RSI /RSI(a) but it takes by default the actual scrip instead of the "foreign" scrips..

Any idea how to do it..

@vince this KB article may be useful:

Indicators based on user values rather than standard OHLC prices

(But RSIa() allows you to specify the array to use). See also the comments to better understand how it works in case you decide to do something similar.