fix: align rasterized artists with get_extent (#742)

Author: timtreis

src/spatialdata_plot/pl/_datashader.py

@@ -730,9 +730,10 @@ def _ax_show_and_transform(
     norm: Normalize | None = None,
     interpolation: str | None = None,
 ) -> matplotlib.image.AxesImage:
-    # ``extent`` uses mpl's pixel-grid convention; world placement happens via
-    # ``set_transform(trans_data)`` afterwards.
-    image_extent = (-0.5, array.shape[1] - 0.5, array.shape[0] - 0.5, -0.5)
+    # Pixel-edge extent [0, W] x [0, H], matching get_extent (which sets the axis limits)
+    # and the affine's data box (placement is via set_transform below). mpl's default
+    # pixel-center extent (-0.5, W-0.5, ...) offsets by half a pixel, amplified by the affine.
+    image_extent = (0.0, array.shape[1], array.shape[0], 0.0)
     # ``alpha`` is applied only when no cmap is set, so RGBA arrays already
     # carrying per-pixel alpha (e.g. datashader output) are not double-attenuated.
     imshow_kwargs: dict[str, Any] = {"zorder": zorder, "extent": image_extent, "norm": norm}

src/spatialdata_plot/pl/render.py

@@ -2400,7 +2400,16 @@ def _draw_labels(
         # non-linear norm (LogNorm/PowerNorm). Display the RGB without a norm and build the
         # continuous colorbar mappable separately from the resolved norm (mirrors the outline path),
         # so the colorbar reflects the real norm subclass.
-        img = ax.imshow(labels, rasterized=True, alpha=alpha, origin="lower", zorder=render_params.zorder)
+        # Pixel-edge extent (0, W, 0, H) matching get_extent/the affine box; mpl's default
+        # pixel-center extent would offset labels half a pixel. Order follows origin="lower".
+        img = ax.imshow(
+            labels,
+            rasterized=True,
+            alpha=alpha,
+            origin="lower",
+            extent=(0.0, labels.shape[1], 0.0, labels.shape[0]),
+            zorder=render_params.zorder,
+        )
         img.set_transform(trans_data)
         if color_spec.is_categorical:
             return img