My plot is empty...?

Hello. I am trying to create a plot containing data from multiple files. I have producecd a code that seems to be running (no errors come up when trying to run it). However, the plot is blank. The code will create a figure window and a plot with axes, titles and everything, but it does not display my data. I have left my code below, does someone spot any mistake or where I could have gone wrong?

%Find and sort files from directory
P = 'C:/Users.../' ; %Working directory (my directory, it is the correct one I have triple checked)
S = dir (fullfile (P , 'PowerMat_*.mat')) ; %All of the files have the same structure for the name. They are PowerMat_xx_xx_xx... .mat (xx are numbers, they are a pretty big list of different parameters that vary with each simulationseparated by _)... not sure this is the correct syntax to recall them?
S = natsortfiles (S) ; %Sort folders / file names into natural order 

%Set conditions for plot
x = 15 : 1 : 50 ; %Array of z values on zmin_15 (from 15 to 50 in steps of 1) for x axis
n = 39 ; %Nº of column to read of the PowerMat (each column is a w value)

%Start plot
figure(1) ;
hold on ;

for k = 1 : numel (S)
    fn = S(k).name ;   % filename :  used for legend
    fn = strrep (fn , ' ' , ' ') ;% replace _ by space for better legend rendering 
    leg{k} = fn ;  % store in cell array
    
    % load data 
    F = fullfile (S(k).folder , S(k).name) ;
    data = load (F) ; % is structure
    names = fieldnames (data) ; % get fieldnames
    y = data.(names{1}) ;  % your data (assumed in first fieldname)
    y = y (: , n) ;    %Selecting column n on data
    plot (x , y)
    
end

set (gca , 'YScale' , 'log') %Sets y axis to be logarithmic scale. Comment for linear scale
title ('title') ;
xlabel ('x label') ;
ylabel ('y label') ;

hold off

[Johan]

You are plotting in log scale, have you checked that your data is positive ? It looks like Matlab plot nothing in log scale if you have less than 2 consecutive points and you are ploting lines.

x = linspace(1,10);
y = x-x(end-1); 
y(5) = 10; 
plot(x,y)
set(gca,'YScale','log')

plot(x,y,'*')

Warning: Negative data ignored

set(gca,'YScale','log')

Warning: Negative data ignored

[Agustina Magarinos-Rodriguez]

All of my data is positive. What I have is a folder with a bunch of files. From each file the code should be reading all rows of data from a specific column and I should have 36 consecutive datapoints being read in each file. Whatever those points are they should be plotted as a line in log scale for the sake of accessibility and making the plots easier to analyse. I have tried leaving it in a linear scale but the data won't show either.

Answer 1

[Image Analyst]

This should really be a FAQ since it's asked so often. You're probably plotting a single data point and you didn't call "hold on" so the next time it goes to call plot, it blows away the previously plotted point.

To fix, either put hold on

plot(x, y, 'b.', 'MarkerSize', 20);
grid on;
hold on;

or call plot after the loop and pass it the whole vectors.

If you have any more questions, then attach your data (mat files) and code to read it in with the paperclip icon after you read this:

TUTORIAL: How to ask a question (on Answers) and get a fast answer

[Agustina Magarinos-Rodriguez]

I do have the hold on and hold off command in the code starting before the for loop and ending at the very end of the code. It seems to not recognise the y variable when running it now?

[Image Analyst]

Did you overlook the part where I said:

If you have any more questions, then attach your data (mat files) and code to read it in with the paperclip icon after you read this:

TUTORIAL: How to ask a question (on Answers) and get a fast answer

Answer 2

[Torsten]

Change

x = 15 : 1 : 50 ;

to

x = (15 : 1 : 50).' ;

[Agustina Magarinos-Rodriguez]

That didn't work.

[Torsten]

x and y must both be column vectors. So that's at least a mistake in your code.

Before plotting you should have a look at y whether it's a column vector with the same number of elements as x and without NaN, +/-Inf or negative values. For this, delete the ";" behind

y = y (: , n) ; %Selecting column n on data

[Agustina Magarinos-Rodriguez]

x is an array that I defined in the code itself and y I know is a column vector that I can see in the file. I thought on the code I also made y to be what I defined it to be...? So the code says that y will be an array of all of the rows within column 'n' which is another number I specified earlier in the code and this I though would make y a vector? Also, all of the values are positive integers with no sign, some of there are values such as '1.32e+05' so exponents.

[Image Analyst]

You still haven't attached 'PowerMat_*.mat'. We'll check back later for it.

[Agustina Magarinos-Rodriguez]

All the files have been added now. Sorry and thank you

Answer 3

[Image Analyst]

I ran your code, attached, and it ran fine: