How do I calculate the area under a curve?

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SS
SS 2020-5-14
评论： Image Analyst 2020-5-16
I have a reading curve from some experimental observations. I want to calculate the area under the curve - between customized X and Y values. I want it in between X=[-0.9,+0.9] and Y=[0.3,0.45]. How can I do it? I am attaching the figure for reference.

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Image Analyst
Image Analyst 2020-5-14
编辑：Image Analyst 2020-5-14
Did you try
indexes = X >= -0.9 & X <= 0.9;
area = trapz(X(indexes), Y(indexes))
But what do you mean with those Y limits? That would just be a rectangular box under the curve.
  2 个评论
SS
SS 2020-5-16
编辑：SS 2020-5-16
Thanks for your reply. I have enclosed that graph just an example. My experimental system gives some reading curve which can also be imported to MATLAB (as a fig file). I don't have specific X and Y coordinates (data points).
How is it possible to calculate the area under the curve of an already existing figure?
Image Analyst
Image Analyst 2020-5-16
Yes, but you'd have to somehow extract the x and y out of the figure. I never do that but I know it's possible. How did the figure get made in the first place? Why do you not have the actual coordinates in your script, function, or in a mat file???

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