[X-axis / Legend] Add option for alphabetical order of categories

[mastropi]

When generating a Group type of graph, allow the option of showing the group values in their natural (alphabetical) order, instead of a rank order (based typically on count).

I was able to simulate this behaviour by creating a dummy field in the dataset equal to 0 in all rows and order by the Max of that field. However, this is cumbersome for the user, and still only works for the ordering of X-axis, not for the order of legend labels of a categorical field.

Use case: the grouping variable is an ordinal field, such as a numeric value, or age range (18-25, 26-35, ...), etc.