Allow passing in Red, Green, Blue grids into grdimage

Using the sequence_space converter to allow for multiple grid inputs into `grdimage`. Useful when users want to make an RGB image plot e.g. from Landsat images (Bands 4,3,2).

Author: weiji14

pygmt/base_plotting.py

@@ -288,22 +288,23 @@ def grdcontour(self, grid, **kwargs):
 
     @fmt_docstring
     @use_alias(R="region", J="projection", W="pen", B="frame", I="shading", C="cmap")
-    @kwargs_to_strings(R="sequence")
+    @kwargs_to_strings(R="sequence", grid="sequence_space")
     def grdimage(self, grid, **kwargs):
         """
         Project grids or images and plot them on maps.
 
         Takes a grid file name or an xarray.DataArray object as input.
+        Alternatively, pass in a list of red, green, blue grids to be imaged.
 
         Full option list at :gmt-docs:`grdimage.html`
 
         {aliases}
 
         Parameters
         ----------
-        grid : str or xarray.DataArray
+        grid : str, xarray.DataArray or list
             The file name of the input grid or the grid loaded as a DataArray.
-
+            A list of red, green, blue grids can also be provided instead.
         """
         kwargs = self._preprocess(**kwargs)
         kind = data_kind(grid, None, None)

pygmt/tests/baseline/test_grdimage_rgb_files.png

@@ -41,6 +41,14 @@ def test_grdimage_file():
     return fig
 
 
+@pytest.mark.mpl_image_compare
+def test_grdimage_rgb_files():
+    "Plot an image using Red, Green, and Blue file inputs"
+    fig = Figure()
+    fig.grdimage(grid=["@earth_relief_60m", "@earth_relief_60m", "@earth_relief_60m"])
+    return fig
+
+
 def test_grdimage_fails():
     "Should fail for unrecognized input"
     fig = Figure()