MATLAB: Making a contour plot from x and y data

Question

I have a two column vector containing x and y values. The y values have a high accuracy (many decimal points).

x ranges from 0 to 50000 and y from 0 to 14.

I need to use contourf to make a 2d representation of this data.

How can this be done?

Answer 1

The bivariate density can be computed using histcounts2. You'll need to specify either the number of bins or the bin edges for the x and y variables using one of the syntaxes below.

• [N,Xedges,Yedges] = histcounts2(X,Y,nbins)
• [N,Xedges,Yedges] = histcounts2(X,Y,Xedges,Yedges)

Here's what the solution will look like. xy is your nx2 matrix of [x,y] values.

[N,Xedges,Yedges] = histcounts2(xy(:,1), xy(:,2), 5);
% or use [N,Xedges,Yedges] = histcounts2(X,Y,Xedges,Yedges);
% Compute bin centers
Xcnt = Xedges(2:end) - abs(diff(Xedges(1:2))/2); 
Ycnt = Yedges(2:end) - abs(diff(Yedges(1:2))/2); 
figure()
contour(Xcnt,Ycnt, N)
% show bins
xlim = [min(Xedges),max(Xedges)]; 
ylim = [min(Yedges),max(Yedges)]; 
arrayfun(@(x)xline(x,'Color',[.8 .8 .8]),Xedges)
arrayfun(@(y)yline(y,'Color',[.8 .8 .8]),Yedges)
% colorbar
cb = colorbar(); 
ylabel(cb,'density')