tiny-plot: changes to scatter plot legend sorting order

[AlexTate]

Groups in scatter_by_dge_class are currently plotted so that groups with fewer points are rendered on top of the stack. The legend's sorting order reflects this, which isn't ideal because point counts can change from condition to condition making the ordering inconsistent between plots for the same experiment. It would be better if class labels were displayed in natural sort order.

Python's sorting functions sort by individual ASCII values, which is great for data but isn't the most human-friendly.

On the other hand, natural sort order:

• Does not differentiate between upper and lowercase letters
• Considers entire numbers rather than the ASCII values of the individual characters that comprise the number

Example plots in doc/tiny-plot.md will need to be updated as well.

[AlexTate]

Reopening this issue to apply natural sorting order to the axis labels in class_charts