find coordinate(-s) on a plot

I have the plot you see above (plotted from data of two vectros X and Y). And I would like to find the coordinates x where y =10. Usually if the y value appears only one time, then I can use the interp1 function.

But what about when this y value belongs to 2 or more x coordinates?

[dpb]

Look for crossing the threshold -- hint: what happens in sign(diff(y)-threshold) when the crossing is rising or falling?

[darova]

I like polyxpoly() function of intersections from fileexchange

Answer 1

[Nishant Gupta]

Hi Argy,

You can use logical indexing to find the x-ordinates corresponding to y = 10 as follows:

x = [1 2 3 4 5];      % x vector
y = [15 10 25 10 5];  % y vector
plot(x,y);
hold on;
yline(10); % plot horizontal line at y = 10
idx = find(y==10);
for i = 1:numel(idx)
 xline(x(idx(i)));  % to get vertical line at all x-ordinates corresponding to y = 10
end
hold off;

[darova]

What if there is no y=10 (y=10.1)