Figure.plot: Add alias "fill_between" for option -M (#3169)

Author: yvonnefroehlich

pygmt/src/plot.py

@@ -29,6 +29,7 @@
     I="intensity",
     L="close",
     N="no_clip",
+    M="fill_between",
     S="style",
     W="pen",
     Z="zvalue",
@@ -178,6 +179,10 @@ def plot(  # noqa: PLR0912, PLR0913
     $fill
         *fill* can be a 1-D array, but it is only valid if using ``x``/``y``
         and ``cmap=True`` is also required.
+    fill_between : str
+        [**c**\|\ **s**][**+l**\ *seclabel*][**+g**\ *fill*][**p**\ *pen*]
+        [**+r**\ *pen*][**+y**\ [*level*]].
+        Fill the middle area between two curves :math:`y_0(x)` and :math:`y_1(x)`.
     intensity : float, bool, or 1-D array
         Provide an *intensity* value (nominally in the -1 to +1 range) to
         modulate the fill color by simulating illumination. If using