Figure.subplot: Migrate the 'margins' parameter to the new alias system (#4174)

Co-authored-by: Yvonne Fröhlich <94163266+yvonnefroehlich@users.noreply.github.com>

Author: seisman

pygmt/src/subplot.py

@@ -6,7 +6,7 @@
 from collections.abc import Sequence
 from typing import Literal
 
-from pygmt.alias import AliasSystem
+from pygmt.alias import Alias, AliasSystem
 from pygmt.clib import Session
 from pygmt.exceptions import GMTInvalidInput, GMTValueError
 from pygmt.helpers import (
@@ -25,16 +25,16 @@
     A="autolabel",
     B="frame",
     C="clearance",
-    M="margins",
     SC="sharex",
     SR="sharey",
     T="title",
 )
-@kwargs_to_strings(Ff="sequence", Fs="sequence", M="sequence")
+@kwargs_to_strings(Ff="sequence", Fs="sequence")
 def subplot(
     self,
     nrows=1,
     ncols=1,
+    margins: float | str | Sequence[float | str] | None = None,
     projection: str | None = None,
     region: Sequence[float | str] | str | None = None,
     verbose: Literal["quiet", "error", "warning", "timing", "info", "compat", "debug"]
@@ -54,6 +54,7 @@ def subplot(
 
     {aliases}
        - J = projection
+       - M = margins
        - R = region
        - V = verbose
 
@@ -108,21 +109,20 @@ def subplot(
         the main map plotting but can be accessed by methods that plot
         scales, bars, text, etc.
     {projection}
-    margins : str or list
-        This is margin space that is added between neighboring subplots (i.e.,
-        the interior margins) in addition to the automatic space added for tick
-        marks, annotations, and labels. The margins can be specified as either:
+    margins
+        Margin space that is added between neighboring subplots (i.e., the interior
+        margins) in addition to the automatic space added for tick marks, annotations,
+        and labels. The margins can be specified as either:
 
         - a single value (for same margin on all sides). E.g. ``"5c"``.
-        - a pair of values (for setting separate horizontal and vertical
-          margins). E.g. ``["5c", "3c"]``.
-        - a set of four values (for setting separate left, right, bottom, and
-          top margins). E.g. ``["1c", "2c", "3c", "4c"]``.
+        - a pair of values (for separate horizontal and vertical margins). E.g.,
+          ``("5c", "3c")``.
+        - a set of four values (for separate left, right, bottom, and top margins).
+          E.g., ``("1c", "2c", "3c", "4c")``.
 
-        The actual gap created is always a sum of the margins for the two
-        opposing sides (e.g., east plus west or south plus north margins)
-        [Default is half the primary annotation font size, giving the full
-        annotation font size as the default gap].
+        The actual gap created is always a sum of the margins for the two opposing sides
+        (e.g., east plus west or south plus north margins) [Default is half the primary
+        annotation font size, giving the full annotation font size as the default gap].
     {region}
     sharex : bool or str
         Set subplot layout for shared x-axes. Use when all subplots in a column
@@ -172,7 +172,9 @@ def subplot(
         msg = "Please provide either one of 'figsize' or 'subsize' only."
         raise GMTInvalidInput(msg)
 
-    aliasdict = AliasSystem().add_common(
+    aliasdict = AliasSystem(
+        M=Alias(margins, name="margins", sep="/", size=(2, 4)),
+    ).add_common(
         J=projection,
         R=region,
         V=verbose,