Add auto-legend feature to grdcontour and contour (#568)

Add label argument to Figure.grdcontour() and Figure.contour().

Author: seisman

pygmt/base_plotting.py

@@ -226,6 +226,7 @@ def colorbar(self, **kwargs):
         S="resample",
         U="timestamp",
         W="pen",
+        l="label",
     )
     @kwargs_to_strings(R="sequence", L="sequence", A="sequence_plus")
     def grdcontour(self, grid, **kwargs):
@@ -275,6 +276,13 @@ def grdcontour(self, grid, **kwargs):
         {G}
         {U}
         {W}
+        label : str
+            Add a legend entry for the contour being plotted. Normally, the
+            annotated contour is selected for the legend. You can select the
+            regular contour instead, or both of them, by considering the label
+            to be of the format [*annotcontlabel*][/*contlabel*]. If either
+            label contains a slash (/) character then use ``|`` as the
+            separator for the two labels instead.
         """
         kwargs = self._preprocess(**kwargs)
         kind = data_kind(grid, None, None)
@@ -568,6 +576,7 @@ def plot(self, x=None, y=None, data=None, sizes=None, direction=None, **kwargs):
         W="pen",
         L="triangular_mesh_pen",
         i="columns",
+        l="label",
         C="levels",
     )
     @kwargs_to_strings(R="sequence", i="sequence_comma")
@@ -619,6 +628,13 @@ def contour(self, x=None, y=None, z=None, data=None, **kwargs):
         skip : bool or str
             Skip input points outside region ``'[p|t]'``
         {W}
+        label : str
+            Add a legend entry for the contour being plotted. Normally, the
+            annotated contour is selected for the legend. You can select the
+            regular contour instead, or both of them, by considering the label
+            to be of the format [*annotcontlabel*][/*contlabel*]. If either
+            label contains a slash (/) character then use ``|`` as the
+            separator for the two labels instead.
 
         """
         kwargs = self._preprocess(**kwargs)