Figure.savefig: Support saving figures in PPM (.ppm) format (#2771)

* Figure.savefig: Support PPM (.ppm) format
* Sort formats alphabetically
* Improve the docstrings to clarify raster and vector image formats

pygmt/figure.py

@@ -255,20 +255,27 @@ def savefig(
         self, fname, transparent=False, crop=True, anti_alias=True, show=False, **kwargs
     ):
         """
-        Save the figure to a file.
+        Save the figure to an image file.
 
-        Supported file formats and their extensions:
+        Supported image formats and their extensions:
+
+        **Raster image formats**
 
-        - PNG (``.png``)
-        - JPEG (``.jpg`` or ``.jpeg``)
-        - PDF (``.pdf``)
         - BMP (``.bmp``)
-        - TIFF (``.tif``)
+        - JPEG (``.jpg`` or ``.jpeg``)
         - GeoTIFF (``.tiff``)
+        - PNG (``.png``)
+        - PPM (``.ppm``)
+        - TIFF (``.tif``)
+
+        **Vector image formats**
+
         - EPS (``.eps``)
-        - KML (``.kml``)
+        - PDF (``.pdf``)
 
-        For KML format, a companion PNG file is also generated.
+        Beside the above formats, you can also save the figure to a KML file
+        (``.kml``), with a companion PNG file generated automatically. The KML
+        file can be viewed in Google Earth.
 
         You can pass in any keyword arguments that
         :meth:`pygmt.Figure.psconvert` accepts.
@@ -284,11 +291,10 @@ def savefig(
         crop : bool
             If ``True``, will crop the figure canvas (page) to the plot area.
         anti_alias: bool
-            If ``True``, will use anti-aliasing when creating raster images
-            (BMP, PNG, JPEG, TIFF, and GeoTIFF). More specifically, it passes
-            the arguments ``"t2"`` and ``"g2"`` to the ``anti_aliasing``
-            parameter of :meth:`pygmt.Figure.psconvert`. Ignored if creating
-            vector graphics.
+            If ``True``, will use anti-aliasing when creating raster images.
+            More specifically, it passes the arguments ``"t2"`` and ``"g2"``
+            to the ``anti_aliasing`` parameter of
+            :meth:`pygmt.Figure.psconvert`. Ignored if creating vector images.
         show: bool
             If ``True``, will open the figure in an external viewer.
         dpi : int
@@ -301,14 +307,15 @@ def savefig(
         """
         # All supported formats
         fmts = {
-            "png": "g",
-            "pdf": "f",
-            "jpg": "j",
             "bmp": "b",
             "eps": "e",
+            "jpg": "j",
+            "kml": "g",
+            "pdf": "f",
+            "png": "g",
+            "ppm": "m",
             "tif": "t",
             "tiff": None,  # GeoTIFF doesn't need the -T option
-            "kml": "g",
         }
 
         fname = Path(fname)

pygmt/tests/test_figure.py

@@ -83,7 +83,7 @@ def test_figure_savefig_exists():
     fig = Figure()
     fig.basemap(region="10/70/-300/800", projection="X3i/5i", frame="af")
     prefix = "test_figure_savefig_exists"
-    for fmt in "png pdf jpg jpeg bmp eps tif PNG JPG JPEG Png".split():
+    for fmt in "bmp eps jpg jpeg pdf png ppm tif PNG JPG JPEG Png".split():
         fname = ".".join([prefix, fmt])
         fig.savefig(fname)