chore(combine): warn about vector formats and test against jpg

cosmoplots/concat.py

@@ -2,6 +2,7 @@
 
 # `Self` was introduced in 3.11, but returning the class type works from 3.7 onwards.
 from __future__ import annotations
+import warnings
 
 import pathlib
 import subprocess
@@ -155,6 +156,12 @@ def _check_params_before_save(
                 if output.name.endswith(self._ft)
                 else output.with_suffix(self._ft)
             )
+        if self._ft in [".eps", ".pdf"]:
+            warnings.warn(
+                "The ImageMagick `convert` command does not work well with vector"
+                " formats. Consider combining the plots directly using matplotlib,"
+                " or change to a different format, such as 'png' or 'jpg'.",
+            )
         if not self._output.parents[0].exists():
             raise FileNotFoundError(
                 f"The file path {self._output.parents[0]} does not exist."

tests/test_concat.py

@@ -85,26 +85,26 @@ def test_combine_ft(tmp_path: pathlib.Path) -> None:
 
     def _combine() -> None:
         plot()
-        plt.savefig(tmp_path / "test1.eps")
+        plt.savefig(tmp_path / "test1.jpg")
         plot()
-        plt.savefig(tmp_path / "test2.eps")
+        plt.savefig(tmp_path / "test2.jpg")
         plot()
-        plt.savefig(tmp_path / "test3.eps")
+        plt.savefig(tmp_path / "test3.jpg")
         plot()
-        plt.savefig(tmp_path / "test4.eps")
+        plt.savefig(tmp_path / "test4.jpg")
         cosmoplots.combine(
-            tmp_path / "test1.eps",
-            tmp_path / "test2.eps",
-            tmp_path / "test3.eps",
-            tmp_path / "test4.eps",
-        ).in_grid(w=2, h=2).save(tmp_path / "out.eps")
+            tmp_path / "test1.jpg",
+            tmp_path / "test2.jpg",
+            tmp_path / "test3.jpg",
+            tmp_path / "test4.jpg",
+        ).in_grid(w=2, h=2).save(tmp_path / "out.jpg")
 
     if platform == "win32":
         with pytest.raises(ChildProcessError):
             _combine()
     else:
         _combine()
-        first_img = tmp_path / "out.eps"
+        first_img = tmp_path / "out.jpg"
         assert first_img.exists()
 
 def test_in_grid_not_specified(tmp_path: pathlib.Path) -> None: