Figure 2. The normalized mutual information, 
, between reference clusters, 
, and the number of clusters, 
, obtained by hierarchical clustering for three data-sets 
(left), 
(right) and 
(bottom). 
for each 
has been generated by sampling the data sets 
, where 
(data set 
).
The total number of descriptors equals 919. They belong to 6 different categories which are as follows: connectivity indices (24), edge adjacency indices (301), topological indices (57), walk path counts (28), information indices (40) and 2D Matrix-based (469).