How to calculate the area under two peaks?

I am trying to calculate the area under two peaks before and after background subtraction.

My code currently looks like this (example data is attached, in .txt form):

%Import the data in from excel
num = importdata('i22-569960_example.dat')

%Smooth data
data = smoothdata(num)

%Calculate area of full graph
x = data(:,1);
y = data(:,2);

[~,imax] = max(y);

area_tot = trapz(x,y)


%Baseline correction
B = [x([1 end]) ones(2,1)] \ y([1 end]);
ydt = [x ones(size(y))] * B;
ybc = y-ydt;                                            % Baseline Corrected
figure
plot(x, ybc)
grid

%Calculate area under peaks
[pks, locs, w, p] = findpeaks(x, ybc)
 

As you can see, I have managed to calculate the total area under the peaks, however I am having issues calculating the area under the peak with findpeaks following this. This is the error code:

Error using findpeaks
Expected X to be strictly increasing.

Error in findpeaks>parse_inputs (line 236)
      validateattributes(Xin1,{'numeric'},{'real','finite','vector','increasing'},'findpeaks','X');

Error in findpeaks (line 135)
  = parse_inputs(isInMATLAB,Yin,varargin{:});

  Error in OrientationScript (line 25)
[pks, locs, w, p] = findpeaks(x, ybc)
 

I'm not quite sure why findpeaks won't work with my data, as my X axis is increasing . I have also looked at using trapz, but I am less familar with that.

Answer 1

[Walter Roberson]

y goes before x in findpeaks() call.

filename = 'https://www.mathworks.com/matlabcentral/answers/uploaded_files/1098535/i22-569960_example.txt';
num = readmatrix(filename);
data = smoothdata(num);
x = data(:,1);
y = data(:,2);

issorted(x)

ans = logical
   1

[~,imax] = max(y);
area_tot = trapz(x,y)

area_tot = 2.4505e+04

%Baseline correction
B = [x([1 end]) ones(2,1)] \ y([1 end]);
ydt = [x ones(size(y))] * B;
ybc = y-ydt;                                            % Baseline Corrected
figure
plot(x, ybc)
grid

%Calculate area under peaks
[pks, locs, w, p] = findpeaks(ybc, x)

pks = 2×1

36.6826 36.0098

locs = 2×1

77.7273 265.9091

w = 2×1

97.9142 83.9262

p = 2×1

36.6826 33.3195

[Sam]

Oh wow, that was a very easy fix! How on earth did I miss that?!

Thank you so much!