How plot a matrix for Naive Bayes classifier?

The heatmap function was introduced in 2017a. The imagesc function will work though. To display the numeric counts you will require a little more work with the text function requiring the location of the text and the x-y data points where the text should appear.

A = randi([0,20],5,5);
imagesc(A);

% Find the x, y locations for the text
x = 1:size(A,2);
y = 1:size(A,1);
% Grid of X,Y locations:
[X,Y] = meshgrid(x,y);

% Convert numbers in matrix to cell array of chars
c = cellfun(@num2str,num2cell(A),'UniformOutput',false);
% Palce text labels at X,Y lcoations:
text(X(:),Y(:),c(:),'HorizontalAlignment','center');
% Add a colorbar
colorbar

This is basically what heatmap does by default, so upgrading makes things quite a bit easier.