test_builder_image: fixed center_on test

Author: PierreRaybaut

plotpy/builder.py

@@ -1257,8 +1257,7 @@ def image(
         assert data.ndim == 2, "Data must have 2 dimensions"
         if pixel_size is None:
             assert center_on is None, (
-                "Ambiguous parameters: both `center_on`"
-                " and `xdata`/`ydata` were specified"
+                "Argument `pixel_size` must be specified when `center_on`"
             )
             xmin, xmax = xdata
             ymin, ymax = ydata

plotpy/tests/unit/test_builder_image.py

@@ -80,7 +80,7 @@ def test_builder_image_pixel_size(qtbot, pixel_size):
 
 @pytest.mark.parametrize("center_on", [None, [1.0, 3.0]])
 def test_builder_image_center_on(qtbot, center_on):
-    item = _make_image(center_on=center_on)
+    item = _make_image(center_on=center_on, pixel_size=(1.0, 1.0))
     show_items_qtbot(qtbot, [item])