Dynamically Choosing an indicator and its plots

I want to write an add-on that trades based on the proximity to any plotted-value of any 'available' indicators. For ex. I can set it up to trade when RSI is approaching 50 in any direction.. I should be able to choose which indicator and which plot I want to select to do the "approach" trade.. Hope what I am describing makes sense.

Now - how do I implement the dynamic selection of indicator and its plot values (similar to how you can do in strategy builder conditions)?

Can someone provide a code example?

Thanks