how to add trend line and output max and min of trend line

Hi all,

I wrote this function:

function [On_Balance_Volume] = OBV(SXXPIndex,PX_VOLUME)

    %%OBV Script

    On_Balance_Volume = onbalvol(SXXPIndex,PX_VOLUME);
    plot(On_Balance_Volume)
    title('On-Balance Volume for S&P')

i would like to add to this function trend line and output the minimum and maximum of the function. can someone advise?