How to generate Heat map/colormap of a dynamic matrix

I have a matrix with 20 rows and the number of columns in the matrix are fixed i.e. first row can have 10 elements, second can have 13, third can have 8 and so on. So it is not completely a matrix. Each element in a matrix has two properties - word and its frequency. Ex: matrix[0][1] = {"abc",10}

I need to create a gray scale heat map of this matrix based on the frequency of each word. I am also not sure how to store this data in MATLAB, what data structure should I use for this? Will normal matrix work? Please any help is appreciated. Thank you.