Add this code after the line N=size(M); (you don't need N in this code anymore)
% round to the nearest integer
S1_x=round(S(1,1));
S1_y=round(S(1,2));
% find the x-index of the black point to the left of the click location
i1 = find(M(S1_y,(1:S1_x))==0, 1, 'last');
% find the x-index of the black point to the right of the click location
i2 = S1_x+find(M(S1_y,(S1_x+1:end))==0, 1, 'first');
% plot a line between the two points
plot([i1,i2], [S1_y,S1_y], 'm-.');
hold on
