Implement default position/box for legend (#359)

Checks for presence of position/D arg before assigning default position ("JTR+jTR+o0.2c") and box ("+gwhite+p1p").

Author: liamtoney

pygmt/base_plotting.py

@@ -621,7 +621,7 @@ def image(self, imagefile, **kwargs):
     @fmt_docstring
     @use_alias(R="region", J="projection", D="position", F="box")
     @kwargs_to_strings(R="sequence")
-    def legend(self, spec=None, **kwargs):
+    def legend(self, spec=None, position="JTR+jTR+o0.2c", box="+gwhite+p1p", **kwargs):
         """
         Plot legends on maps.
 
@@ -644,13 +644,23 @@ def legend(self, spec=None, **kwargs):
         {R}
         position (D) : str
             ``'[g|j|J|n|x]refpoint+wwidth[/height][+jjustify][+lspacing][+odx[/dy]]'``
-            Defines the reference point on the map for the legend.
+            Defines the reference point on the map for the legend. By default, uses
+            'JTR+jTR+o0.2c' which places the legend at the top-right corner inside
+            the map frame, with a 0.2 cm offset.
         box (F) : bool or str
             ``'[+cclearances][+gfill][+i[[gap/]pen]][+p[pen]][+r[radius]][+s[[dx/dy/][shade]]]'``
             Without further options, draws a rectangular border around the
-            legend using **MAP_FRAME_PEN**.
+            legend using **MAP_FRAME_PEN**. By default, uses '+gwhite+p1p' which draws
+            a box around the legend using a 1 point black pen and adds a white background.
         """
         kwargs = self._preprocess(**kwargs)
+
+        if "D" not in kwargs:
+            kwargs["D"] = position
+
+            if "F" not in kwargs:
+                kwargs["F"] = box
+
         with Session() as lib:
             if spec is None:
                 specfile = ""

pygmt/tests/baseline/test_legend_default_position.png

@@ -28,6 +28,22 @@ def test_legend_position():
     return fig
 
 
+@pytest.mark.mpl_image_compare
+def test_legend_default_position():
+    """
+    Try using the default legend position.
+    """
+
+    fig = Figure()
+
+    fig.basemap(region=[-1, 1, -1, 1], frame=True)
+
+    fig.plot(x=[0], y=[0], style="p10p", label="Default")
+    fig.legend()
+
+    return fig
+
+
 @pytest.mark.mpl_image_compare
 def test_legend_entries():
     """