As you see in the img I have the same legend for the four subplot.
How can I position it only once outside the subplot? Maybe to the right of subplots or to the left or at centre (iterazione means iteration).
Then, I know that I can use a title for each subplot but how can I use just only title for the whole figure?
Only call the legend in subplot(2,2,2), and not the others, and use -1 as the position indicator. That will put it outside (to the right) of the second subplot in the first row.