Smart sorting of the legend for plotting

[EvanBianco]

It would be nice to provide some ways to sort and or group decors in legend. A few fields by which to sort by could be:

• alphabetically on the name
• abundance
• age or stratigraphic order

Folks may also want to manually set the order, by providing a list say.

[kwinkunks]

NB The user effectively can manually set the order now, by building the legend in the desired order.