Plotting rows of points in Matlab

Question

So I'm still getting used to Matlab and am having a bit of trouble with plotting. I have a cell which contains a list of points in each row. I want to plot each row of points in a different colour on the same graph so I can compare them. The catch is that I need to make this work for an unknown number of points and rows (ie the number of points and rows can change each time I run the program).

So for example, I might have my cell array A:

A = {[0,0], [1,2], [3,4]; [0,0] [5,6], [9,2]}

and I want to plot the points in row 1 against their index (so a 3D graph) and then have the points in row 2 on the same graph in a different colour. The rows will always be the same length. (Each row will always have the same number of points). I've tried a few different for loops but just can't seem to get this right.

Any help in sending me in the right direction would be greatly appreciated!

Answer 1

You could just convert it to a matrix and plot it directly:

% Some dummy data - format a little different from your example 
% to allow for different numbers of elements per row
A = {[0,0, 1,2, 3,4]; [0,0, 5,6]};

% Figure out how many columns we need in total
maxLen = max(cellfun(@length, A));
% Preallocate
Amat = NaN(size(A, 1), maxLen);
% Copy data
for n = 1:size(A, 1)
    curA = A{n}; 
    Amat(n, 1:length(curA)) = curA; 
end

% Generate 1:N vector repeated the correct number of times (rows)
x = repmat(1:size(Amat, 2), size(Amat, 1), 1);
plot(x, Amat)

Edit: You mentioned a 3D graph at some point in your post. The above won't plot a 3D graph, so here's something that will:

% Generate Amat as above

% Then:
[X, Y] = meshgrid(1:size(Amat, 1), 1:size(Amat, 2));
surf(X, Y, Amat.'); % OR: plot3(X, Y, Amat.');

I'm not sure this is exactly what you want, but your question is slightly unclear on exactly what kind of graph you want out of this. If you just want coloured lines on your plot, you can use plot3 instead of surf , but IMHO surf will probably give you a clearer plot for this kind of data.

Answer 2

The fact that the number of points and rows can change with each iteration should not pose much of a problem. I would suggest using the size function before your plot loops (size(A,1) and size(A,2)) to get the dimensions of the matrix.

Once you have the size of the matrix, loop through the dimensions and plot the lines on the same plot using holdon, and then finally just make the line color a function of the dimensions as you loop through so that you always have a different line color