Panel A: relationship between the in-degree and the undirected-degree for each node; observe that there is a negative correlation, i.e. in-hubs tend to have very few undirected links and vice versa (insets correspond to covariance between the plotted variables). Panel B: same as Panel A, but for the out-degree of the nodes; in this case, there is a strong correlation between out-hubs and undirected-hubs. Panel C: relationship between out- and in-degree for each node. The plot makes evident that nodes tend to be either in-hubs or out-hubs, as the correlation between the degrees is basically insignificant; moreover, large hubs tend to lay on the axes, i.e. they have a bias to be pure "sources" (horizontal) or "sinks" (vertical). Another way to see this phenomenon is presented in Panel D, where the maximum between the in- and out-degrees is plotted against the absolute value of the difference between the same quantities. As the plot shows, nodes tend to cluster near the identity line, which corresponds to pure sources and sinks.
Cecchi et al. BMC Cell Biology 2007 8(Suppl 1):S5 doi:10.1186/1471-2121-8-S1-S5