fix: honor aspect='auto' in px.imshow for RGB images

plotly.js anchors the y axis of an image trace to its x axis by default, so px.imshow kept square pixels for RGB and RGBA images even with aspect='auto'. Set scaleanchor=False on every y axis in that case.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: jayzhou2309

CHANGELOG.md

@@ -7,6 +7,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 - Fix concurrent first access to lazily initialized graph object properties, which could raise `ValueError("Invalid value")` [[#3441](https://github.com/plotly/plotly.py/issues/3441)], with thanks to @hb1915 for the contribution!
 - Fix `px.sunburst`, `px.treemap` and `px.icicle` listing sectors in a different order on every run when `path` is used with a Polars DataFrame; sectors now follow their order of first appearance for all dataframe backends [[#5765](https://github.com/plotly/plotly.py/issues/5765)], with thanks to @Irahan2 for the contribution!
+- Fix `px.imshow` ignoring `aspect="auto"` for RGB and RGBA images, which kept their pixels square instead of stretching the image to fill the axes [[#5172](https://github.com/plotly/plotly.py/issues/5172)], with thanks to @jayzhou2309 for the contribution!
 
 
 ## [7.1.0] - 2026-09-15

plotly/express/_imshow.py

@@ -631,6 +631,9 @@ def imshow(
     if animation_frame:
         fig.frames = frame_list
     fig.update_layout(layout)
+    if trace["type"] == "image" and aspect == "auto":
+        # plotly.js anchors the y axis of an image trace to its x axis by default
+        fig.update_yaxes(scaleanchor=False)
     # Hover name, z or color
     if binary_string and rescale_image and not np.all(img == img_rescaled):
         # we rescaled the image, hence z is not displayed in hover since it does