Issue with reading data in text files and plotting a graph (Matlab) - matlab

Question

I use the following code to collect data from two text files join them together and then plot them. For some reason, I appear to get two plots rather than 1, I am not sure why this is happening.
load MODES1.dat; % read data into the MODES1 matrix
x1 = MODES1(:,1); % copy first column of MODES1 into x1
y1 = MODES1(:,2); % and second column of MODES1 into y1
load MODES.dat; % read data into the MODES matrix
x = MODES(:,1); % copy first column of MODES into x
y = MODES(:,2); % and second column of MODES into y
% Joining the two sets of data
endx = [x1;x];
endy = [y1;y];
figure(1)
plot(endx,endy)
xlabel('Unique Threshold Strains','FontSize',12);
ylabel('Probabilities of occurrence','FontSize',12);
title('\it{Unique Values versus frequencies of occurrence}','FontSize',16);
Thanks

Answer 1

Your problem is quite a simple one. Matlab's plot command creates a point for each data point defined by the parameters and connects those points in the order they appeared in the first parameter. To get an idea of this behavior, try
x = [0;1;-1;2;-2;3;-3;4;-4;5];
plot(x,x.^2);
You won't get the quadratic function graph you might expect.
To fix this, you must sort you input arrays identically. Sorting one array is simple (sort(endx)), but you want to sort both in the same way. Matlab actually gives you a function to do this, but it only works on matrices, so you need to do some concatenating/seperating:
input = sortrows( [endx endy] );
endx = input(:,1);
endy = input(:,2);
This will sort the rows of the matrix built by putting endy right of endx with respect to the first column (endx). Now your inputs are correctly sorted and the resulting plot should only show one line. (More accurately, one line which does not at some point go back where it came from.)
Another way to achieve this, depending on you actual use case and data origin, would be to build the mean value of both parts of x, so instead of endx = [x1;x];, you'd build endx = mean([x1 x],2);.
Yet another way is to drop the line altogether and go with
plot(endx,endy,'.');
or
plot(endx,endy,'LineStyle','none');
But this is only useful if your data points are very close to each other.