Supposing you had done
plot(x,y)
and that the value you are looking for is ProbeY (an arbitrary variable name)
[ydist, yidx] = sort(abs(y - ProbeY));
Then
x(yidx(1))
is the x for which y(x) is the closest to ProbeY.
Sometimes, though, you only want locations where y(x) is at most ProbeY
ylocs = find(ProbeY >= y);
[ydist, ylocsidx] = sort(ProbeY(ylocs)-y);
closestyidx = ylocs(ylocsidx(1));
and then x(closestyidx) is the x for which y(x) is closest to but does not exceed ProbeY.
