Modules that should take -l for auto-legend building

[PaulWessel]

Description of the desired feature

Looking at all the modules that produce plots, I think this is the list that may need -l:

• contour: Need to resolve how/if to deal with annotated vs regular contours.
• grdcontour: ditto
• grdvector: Place a scaled vector in legend. Alternatively, or possible in addition, to how wiggle plots a scale bar?
• plot/plot3d: Need to allow -l for polygon, plotting a filled/outlined rectangle.
• velo: maybe arrow with error ellipse? Or like grdvector add option to plot a scale
• meca: Could plot using the mega custom symbol in plot.

For contours the proposed option was -l[annotlabel]/[contourlabel] in order to allow one or both lines.
I have excluded modules like coast and wiggle since not likely to need legends (?).

[seisman]

@PaulWessel No need to have "6.1" in the title. You can add this issue to the "6.1.0" milestone.

[PaulWessel]

Slowly learning GitHub...

[maxrjones]

xref #442 (comment), which requests auto-legend support for custom symbols in plot.