How to find the max value of a curve on a graph

I'm trying to find the max value of my G(T) curve(the parabolic one). Not sure how to het the value and make it show up as a marker on the graph

T = [310:1:450];
v = 1;
    V = 10;
    dHrxn = -80000;
    UA = 3600;
    kf0 = 1;
    kc0 = 100;
    T0 = 400;
    ER = 20000;
    CpA = 40;
    kf = exp(-20000*(1./T-1/400));
    kc = 100*exp((80000/1.987)*(1./T-1/400));
    x = 1./(v./(V.*kf)+1+(1./kc));
    R = 400*(T-310);
    R1 = 400*(T-256);
    R2 = 400*(T-233.5);
    R3 = 400*(T-218.2);
    R4 = 400*(T-391);
    R5 = 400*(T-328);
    R6 = 400*(T-346);
    R7 = 400*(T-353.2);
    G = x*-dHrxn;
    
figure
hold all
plot(T,R,T,G,T,R1,T,R2,T,R3,T,R4,T,R5,T,R6,T,R7)
legend('R(T)','G(T)','Ta=250K','Ta=225K','Ta=208K','Ta=400K','Ta=330K','Ta=350K','Ta=358K','Location','NorthWest')
axis([310 450 0 10*10^4])
xlabel('Temperature (K)')
ylabel('G(T) and R(T) (cal/mol)')
title('G(T) and R(T) vs. T')

Answer 1

[KSSV]

If (x,y) are the data points.

[val,idx] = max(y) ; 
plot(x,y)
hold on
plot(x(idx),y(idx),'*r')