How to find the x value corresponding to the half of a curve?

Can anyone help out with the Matlab codes necessary to determine the a, b (y axis) and c (x axis) values in the following graph? Thanks.

Answer 1

[Andrei Bobrov]

S - your signal; time1 - time.

[a,ia] = max(S);
l1 =  [false;diff(S(1:ia)) < 1];
ib = find(l1,1,'last');
b = S(ib);
Csig = (a+b)/2;
Sab = S(ib:ia);
t = time1(ib:ia);
ii = find(Sab < Csig,1,'last');
C = interp1(Sab([ii,ii+1]),t([ii,ii+1]),Csig);