How to evaluate the highest value of the displayed range of an indicator?

I'm writing a pine code and need to evaluate the highest and lowest value of an indictor (like rsi) displayed on the window (shown time range), and if I change the time range it can be updated automatically (like high, avg, low label of price bar that trading view has), anyone knows how can I evaluate the highest value of the displayed range of an indicator?

or how can we get the bar_index of the first and last bar displayed on the tradingview chart?

Thanks

Hello there

To the first question, use this
https://www.tradingview.com/pine-script-reference/v4/#fun_highest

To the second question, bar_index of the first is bar is always 0 and bar_index of the last bar is always 20000 is your a Premium user, otherwise it's 10000