Allow for io.StringIO inputs into legend's spec argument

There is a new 'buffer' data_kind which is
intended for io.StringIO stream data.
This 'buffer' data is passed into `legend`
via an intermediate file created using the
helper tempfile_from_buffer function.

pygmt/base_plotting.py

@@ -4,19 +4,21 @@
 """
 import contextlib
 import csv
+
 import numpy as np
 import pandas as pd
 
 from .clib import Session
 from .exceptions import GMTError, GMTInvalidInput
 from .helpers import (
+    GMTTempFile,
     build_arg_string,
-    dummy_context,
     data_kind,
+    dummy_context,
     fmt_docstring,
-    GMTTempFile,
-    use_alias,
     kwargs_to_strings,
+    tempfile_from_buffer,
+    use_alias,
 )
 
 
@@ -801,10 +803,11 @@ def legend(self, spec=None, position="JTR+jTR+o0.2c", box="+gwhite+p1p", **kwarg
 
         Parameters
         ----------
-        spec : None or str
-            Either None (default) for using the automatically generated legend
-            specification file, or a filename pointing to the legend
-            specification file.
+        spec : None or str or io.StringIO
+            Set to None (default) for using the automatically generated legend
+            specification file. Alternatively, pass in a filename or an
+            io.StringIO in-memory stream buffer pointing to the legend
+            specification text.
         {J}
         {R}
         position : str
@@ -829,13 +832,17 @@ def legend(self, spec=None, position="JTR+jTR+o0.2c", box="+gwhite+p1p", **kwarg
 
         with Session() as lib:
             if spec is None:
-                specfile = ""
+                file_context = dummy_context("")
             elif data_kind(spec) == "file":
-                specfile = spec
+                file_context = dummy_context(spec)
+            elif data_kind(spec) == "buffer":
+                file_context = tempfile_from_buffer(spec)
             else:
-                raise GMTInvalidInput("Unrecognized data type: {}".format(type(spec)))
-            arg_str = " ".join([specfile, build_arg_string(kwargs)])
-            lib.call_module("legend", arg_str)
+                raise GMTInvalidInput(f"Unrecognized data type: {type(spec)}")
+
+            with file_context as fname:
+                arg_str = " ".join([fname, build_arg_string(kwargs)])
+                lib.call_module("legend", arg_str)
 
     @fmt_docstring
     @use_alias(

pygmt/helpers/utils.py

@@ -1,9 +1,10 @@
 """
 Utilities and common tasks for wrapping the GMT modules.
 """
-import sys
+import io
 import shutil
 import subprocess
+import sys
 import webbrowser
 from collections.abc import Iterable
 from contextlib import contextmanager
@@ -62,6 +63,8 @@ def data_kind(data, x=None, y=None, z=None):
 
     if isinstance(data, str):
         kind = "file"
+    elif isinstance(data, io.StringIO):
+        kind = "buffer"
     elif isinstance(data, xr.DataArray):
         kind = "grid"
     elif data is not None:

pygmt/tests/test_legend.py

@@ -1,6 +1,8 @@
 """
 Tests for legend
 """
+import io
+
 import pytest
 
 from .. import Figure
@@ -44,9 +46,6 @@ def test_legend_default_position():
     return fig
 
 
-@pytest.mark.xfail(
-    reason="Baseline image not updated to use earth relief grid in GMT 6.1.0",
-)
 @pytest.mark.mpl_image_compare
 def test_legend_entries():
     """
@@ -73,7 +72,8 @@ def test_legend_entries():
 
 
 @pytest.mark.mpl_image_compare
-def test_legend_specfile():
+@pytest.mark.parametrize("usebuffer", [True, False])
+def test_legend_specfile(usebuffer):
     """
     Test specfile functionality.
     """
@@ -113,7 +113,10 @@ def test_legend_specfile():
         fig = Figure()
 
         fig.basemap(projection="x6i", region=[0, 1, 0, 1], frame=True)
-        fig.legend(specfile.name, position="JTM+jCM+w5i")
+
+        spec = io.StringIO(specfile_contents) if usebuffer else specfile.name
+
+        fig.legend(spec=spec, position="JTM+jCM+w5i")
 
     return fig