The following code

 ## Generate a two-cluster problem
 C1 = randn (100, 2) + 1;
 C2 = randn (100, 2) - 1;
 data = [C1; C2];

 ## Perform clustering
 [idx, centers] = kmeans (data, 2);

 ## Plot the result
 figure
 plot (data (idx==1, 1), data (idx==1, 2), 'ro');
 hold on
 plot (data (idx==2, 1), data (idx==2, 2), 'bs');
 plot (centers (:, 1), centers (:, 2), 'kv', 'markersize', 10);
 hold off

Produces the following figure

Figure 1