Matlab: Plotting multiple points with same coordinates

0
votes

I have some data that I want to plot in MATLAB. The data consists of x and y coordinates of the points that I want. At the moment, I am using the plot function to plot these points.

The data has a lot of points with the same coordinates. Now, what I want is that points with the same coordinates do not plot as a single dot, but a thicker dot.

For example, suppose the points are

1,1

2,1

2,1

3,2

2,1

2,1

1,1

Then, the plot should have a single dot at 3,2, but a thicker dot at 1,1 and even a thicker dot at 2,1

Can someone tell me how to do this?

3
matlabplotmatlab-figure
Don't know any MATLAB way of doing this, but I used to do such things in my own routines by plotting a small circle (circle is adequate for this, although that marker could be anything) and just increasing the radius by one every time the same dot is hit. In the end you get a large filled circle depending on the number of times you "hit" the same place. – Rook

3 Answers

3
votes

You can use a bit of creativity and the scatter function to do this.

First, you need to reduce your data to a list of points, plus a count of the number of points at each location.

For example if you have some data:

xy = [...
    1,1; ...
    2,1; ...
    2,1; ...
    3,2; ...
    2,1; ...
    2,1; ...
    1,1];

Get the unique points, and the unique indexes:

[xyUnique, ignore, ixs] = unique(xy,'rows')

This is pretty painful, but we can count the number of occurrences of each unique row using the ixs vector (there is probably a better way).

counts = zeros(size(xyUnique,1),1);
for ix = 1:size(counts,1);
    counts(ix) = sum(ixs == ix);
end

Now use scatter to make a plot as you want

scatter(...
    xyUnique(:,1), ...  %X values
    xyUnique(:,2), ...  %Y values
    counts*20, ...      %Individual marker sizes, note scale factor to make this visible
    'b', ...            %Marker colors
    'filled');          %I think these look better filled
1
votes

To avoid looping, building on the previous example, try this:

xy = [...
    1,1; ...
    2,1; ...
    2,1; ...
    3,2; ...
    2,1; ...
    2,1; ...
    1,1];

[xyUnique, ignore, ixs] = unique(xy,'rows')

Will result in

xyUnique =
     1     1
     2     1
     3     2

Next, we use the function hist

[nRows, nCols] = size(xyUnique)
xyCount = hist(ixs,nRows)

Which results in 

xyCount =
     2     4     1

Each value of xyCount is number of occurrences of each row of xyUnique.

0
votes

Use the scatter command of the form:

scatter(X,Y,S)

You will have to determine how many times the coordinates are repeated to set the right vector for S.

Description:

scatter(X,Y,S) draws the markers at the specified sizes (S) with a single color. This type of graph is also known as a bubble plot.

S determines the area of each marker (specified in points^2). S can be a vector the same length as X and Y or a scalar. If S is a scalar, MATLAB draws all the markers the same size. If S is empty, the default size is used.

For more information see documentation.