How to sort points for plotting in MATLAB?

Hello! I've got a little question about plotting points. In my project, I'm getting x and y coordinates and plotting them just when I read them from the socket, and I get something like this:

Just after plotting the coordinates I'm indexing them in two different vectors (one for x coordinates and the other for y coordinates). The problem is, when I try to plot them again in another script (or in another figure) the result is not what I expected.

The code I use for plotting the points the second time is:

figure()
for i=1:1000         %1000 is the length of x_vector and y_vector
   plot(x_vector(i), y_vector(i), 'o')
   hold on
   axis([0 1920 0 1080]);
   set(gca, 'ydir', 'reverse', 'xaxislocation', 'top')
end

And this is the result of the previous code:

I guess that the second time I try to plot the two vectors plot is trying to print some linear function through some regression instead of printing the cloud of points I expect.

Thank you

[madhan ravi]

Upload your datas too (x_vector and y_vector).

[Stephan]

Hi,

Why do you use a loop for this?

scatter(x_vector, y_vector, 'or')
axis([0 1920 0 1080]);
set(gca, 'ydir', 'reverse', 'xaxislocation', 'top')

[dpb]

I think we would need to see how the x,y coordinates were stored and the original figure as well, not just the latter.

[madhan ravi]

I second @dpb

[Ángel González]

I first created the two vectors:

x_vector = [];
y_vector = [];

Then, I stored the x and y the eyetracker sends:

x_vector = sort([x_vector x]);
y_vector = sort([y_vector y]);

And the original figure is the first one I posted on the question.

[Ángel González]

I upload the two vectors saved from Matlab.

PS: By using scatter I get a similar result..

[dpb]

x_vector = sort([x_vector x]);
y_vector = sort([y_vector y]);

And that explains it...you sorted the points independent of each other so the association of x,y is broken.

[Ángel González]

@dpb, which solution would you use to storage the x and y values?

Thank you!

[dpb]

Simply store the points as you receive them...you don't show code but

% preallocate
N=npts;
x_vector=zeros(npts,1);
y_vector=x_vector;
% collect, save...
for i=1:npts
  % return x, y, here...
  x_vector(i)=x;
  y_vector(i)=y;
  % do whatever else with x,y during download
  ...
end

presuming you know a priori how many points you're going to collect. If that is unknown, start with a big number and then remove those not needed.

Alternatively, while it's not efficient to code this way, the dynamic allocation as you wrote above is simple but overhead intensive--

x_vector=[]; y_vector=[];
while stilldownloading()
  % get x, y
  x_vector=[x_vector;x];
  y_vector=[y_vector;y];
  ...
end

will keep each x,y pair together.

[Jan]

Maybe replace

x_vector = sort([x_vector x]);
y_vector = sort([y_vector y]);

by

[x_vector, index] = sort([x_vector x]);
yTemp             = [y_vector y];
y_vector          = yTemp(index);

[dpb]

That's calling sort repetitively every iteration...there's no indication of needing them sorted; if do, far more efficient to wait until done collecting and then sort the whole vector once.

Keeping and reordering y based on order of sorted x is, of course, necessry to keep the association between the two.

Alternatively, one could do slightly differently as

xy=[];
while collecting()
  % get x, y
  xy=[xy; [x y]];
  ...
end
xy=sortrows(xy);

and will have x,y in two columns in the one arrray sorted by increasing x

Answer 1

[Ángel González]

Thank you all for helping me. As you said, the problem was how I was storing the coordinates. This comment from @dpb resumed the problem perfectly: https://es.mathworks.com/matlabcentral/answers/416020-how-can-i-plot-points-in-matlab#comment_602802

[dpb]

You're welcome... :)

Answer 2

[Afshin Aghayan]

you can use this code for displaying any data in the form of [x, y, f(x,y)] or data with coordinate

https://www.mathworks.com/matlabcentral/fileexchange/72970-display-data-in-form-of-f-x-y-as-map-and-convert-to-matrix