How to extract data from fnplt plot?

I created two plots using the fnplt function. One plot represents the chord length (c) over the radial position, the other the thickness (t) over the radial position (-> they both have the same x-axis).

The chord and thickness values are calculated via ppmak function (with breaks and coefficients):

chord = ppmak(breaks_chord,coefs_chord,1);
for i = 1:length(breaks_chord)-1
    fnplt(chord,[breaks_chord(i) breaks_chord(i+1)],'k')
    hold on
end

I need to calculate the height h = c*t and plot h against the radial position.

How can I extract the c and t values at specific x-axis values (e.g. x=10, x=20, x=30)?

The functions 'findobj' and 'get' do not work.

Thanks in advance!