% Dummy example of some curve that cross the horizontal line y=10
x = cumsum([0 rand(1,40)]);
y = 12*sin(x);
plot(x,y,'.-');
hl = findobj(gca, '-depth', 1, 'type', 'line');
x=get(hl,'xdata');
y=get(hl,'ydata');
y10 = -10;
i = find((y(1:end-1)-y10).*(y(2:end)-y10) < 0);
ipick = i(1); % select where it cross and where x is (in your graph it has at lzast 2 crossing points)
x10 = interp1(y(ipick:ipick+1),x(ipick:ipick+1),y10)
hold on
xline(x10)
yline(y10)
plot(x10,y10,'ro','Markersize',10)
