Fix illogical test

This had tested what would happen if you wanted to use x/y for a catalog
but had no x/y in the catalog table and didn't expect that to raise an
error.

Author: mwcraig

src/astro_image_display_api/api_test.py

@@ -543,13 +543,10 @@ def test_load_catalog_with_skycoord_no_wcs(self, catalog, data):
 
         # Remove x/y columns from the catalog
         del catalog["x", "y"]
-        self.image.load_catalog(catalog)
-        # Retrieve the catalog and check that the x and y columns are None
-        retrieved_catalog = self.image.get_catalog()
-        assert "x" in retrieved_catalog.colnames
-        assert "y" in retrieved_catalog.colnames
-        assert all(rc is None for rc in retrieved_catalog["x"])
-        assert all(rc is None for rc in retrieved_catalog["y"])
+        with pytest.raises(
+            ValueError, match="Cannot use pixel coordinates without pixel columns"
+        ):
+            self.image.load_catalog(catalog)
 
     def test_load_catalog_with_use_skycoord_no_skycoord_no_wcs(self, catalog, data):
         # Check that loading a catalog with use_skycoord=True but no

src/astro_image_display_api/image_viewer_logic.py

@@ -492,9 +492,16 @@ def load_catalog(
                 x, y = self._wcs.world_to_pixel(coords)
                 to_add[x_colname] = x
                 to_add[y_colname] = y
+                xy = (x, y)
             else:
                 to_add[x_colname] = to_add[y_colname] = None
 
+        if not use_skycoord and xy is None:
+            raise ValueError(
+                "Cannot use pixel coordinates without pixel columns or both "
+                "coordinates and a WCS."
+            )
+
         if coords is None:
             if use_skycoord and self._wcs is None:
                 raise ValueError(
@@ -703,13 +710,15 @@ def get_viewport(
                         "sky coordinates."
                     )
                 else:
-                    center = viewport.wcs.pixel_to_world(
-                        viewport.center[0], viewport.center[1]
-                    )
-                    pixel_scale = proj_plane_pixel_scales(viewport.wcs)[
-                        viewport.largest_dimension
-                    ]
-                    fov = pixel_scale * viewport.fov * u.degree
+                    if center is None:
+                        center = viewport.wcs.pixel_to_world(
+                            viewport.center[0], viewport.center[1]
+                        )
+                    if fov is None:
+                        pixel_scale = proj_plane_pixel_scales(viewport.wcs)[
+                            viewport.largest_dimension
+                        ]
+                        fov = pixel_scale * viewport.fov * u.degree
         else:
             # Pixel coordinates
             if isinstance(viewport.center, SkyCoord):