MATLAB: Integrate a partial area under a plotted curve

area under curveMATLABnumerical integration

Hello every one,
I'm trying to calculate a partial area under a curve.
I've seen some asked questions about the subsject, but still struggling with some points.
I plotted a cuvre using numerical column vectors (electrical current versus time), then using this proposed solution, i tried to calculate the area under the curve:
yi = interp1(x, y, [x1:x2]); % x1 and x2 are the boundaries
shaded_area = trapz([4:5],yi+5)
Where y represents my electrical current vector and x my time vector, which contains by the way negative values, but i don't think that causes any problem.
When i execute the first line, it returns 'Nan' values!! So, I tried other column vectors which are extracted from the former ones (x and y) but i took only the positive values and again it returns NaN.
I thinking about extrapolating problem, but i have non idea about how to start with this issu?
It'll be very helpful if some one could give me some advice. Thank you!
Please find the attached source data file, function allowing importing data and the commands file (in the commands file, i changed the name of the dataset).
Using Matlab_2018b
https://fr.mathworks.com/matlabcentral/answers/314577-how-to-calculate-partial-area-under-a-graph

Best Answer

To get the area, add these lines to your code: 
[TF,Q0] = ischange(ifuse, 'linear', 'Threshold', 1.5E+6);
[vl,vli] = min(ifuse(TF));
[pk,pki] = max(ifuse(TF));
Idx = find(TF);
idxrng = Idx(vli):Idx(pki);
Int_ifuse_pk = trapz(ox_axis1(idxrng), ifuse(idxrng))
figure
plot(ox_axis1,ifuse,'r') 
hold on
% plot(ox_axis1(TF),ifuse(TF),'+g')
patch([ox_axis1(idxrng); flipud(ox_axis1(idxrng))], [ifuse(idxrng); ones(size(ifuse(idxrng)))*ifuse(Idx(vli))], 'k', 'FaceAlpha',0.25)
hold off
([min(ox_axis1) max(ox_axis1)])
text(max(ox_axis1(idxrng)), mean(ifuse(idxrng)), sprintf('\\leftarrow Area = %.3f', Int_ifuse_pk), 'HorizontalAlignment','left')
producing the desired area (as ‘Int_ifuse_pk’), and this plot:
Experiment with this to get different results.
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