autosubmit plots should use natural order to sort entries

I was playing with some workflows for an issue from @froura (#1421 (closed)), and noticed that plotting the splits I get each split appearing in an order with splits starting 10, 11, 12, 13, ..., 1, then 20, 21, ..., 2. This is confusing for users, and displaying in natural order (i.e. 1, 2, 3, ..., 9, 10, 11`, ...) would be more user friendly.

Not sure if for 4.1.11 or later. Maybe something Irene could work on too, to get familiar with the code.