A density plot using 2 coordinates columns and a weight column

Question

I have a matrix with 3 columns. The first two columns are coordinates and the third is weight or intensity.

newmat = [ 27.37  -45.69   14.47
           27.37  -45.68   18.58
           27.37  -45.67   29.05
           27.37  -45.66   51.7
            ...     ...     ... ]

I have already created a scatter plot:

However, I'd like to have something like a density plot (as the second plot here). I have tried to use hist3 function as in here, but I didn't figure out how to take into account the third column - weight.

Answer 1

You could create a matrix from the data in newmat (using the functions sortrows, unique, and accumarray) and plot it as an image:

newmat = sortrows(newmat, [1 2]);  % Sort the first two columns in ascending order
[x, ~, newmat(:, 1)] = unique(newmat(:, 1));     % Make numeric indices for column 1
[y, ~, newmat(:, 2)] = unique(newmat(:, 2));     % Make numeric indices for column 2
M = accumarray(newmat(:, 1:2), newmat(:, 3)).';  % Build the matrix
imagesc(x, y, M);

Here's some sample data similar to your format:

[X, Y] = meshgrid(0:0.1:2, 3:0.1:5);
Z = peaks(21);
newmat = [X(:) Y(:) Z(:)];

And here's the plot the above code produces from that data: