no color in hovertemplate when contrast has been modified

Author: emmanuelle

packages/python/plotly/plotly/express/_imshow.py

@@ -422,21 +422,25 @@ def imshow(
     fig = go.Figure(data=trace, layout=layout)
     fig.update_layout(layout_patch)
     # Hover name, z or color
-    if trace["type"] == "heatmap":
-        hover_name = "z"
-    elif img.ndim == 2:
-        hover_name = "color[0]"
+    if binary_string and not (img_rescaled.max() == img.max()):
+        hovertemplate = "%s: %%{x}<br>%s: %%{y}<extra></extra>" % (
+            labels["x"] or "x",
+            labels["y"] or "y",
+        )
     else:
-        hover_name = "color"
-    fig.update_traces(
-        hovertemplate="%s: %%{x}<br>%s: %%{y}<br>%s: %%{%s}<extra></extra>"
-        % (
+        if trace["type"] == "heatmap":
+            hover_name = "z"
+        elif img.ndim == 2:
+            hover_name = "color[0]"
+        else:
+            hover_name = "color"
+        hovertemplate = "%s: %%{x}<br>%s: %%{y}<br>%s: %%{%s}<extra></extra>" % (
             labels["x"] or "x",
             labels["y"] or "y",
             labels["color"] or "color",
             hover_name,
         )
-    )
+    fig.update_traces(hovertemplate=hovertemplate)
     if labels["x"]:
         fig.update_xaxes(title_text=labels["x"])
     if labels["y"]:

packages/python/plotly/plotly/tests/test_core/test_px/test_imshow.py

@@ -285,3 +285,11 @@ def test_imshow_hovertemplate(binary_string):
         fig.data[0].hovertemplate
         == "x: %{x}<br>y: %{y}<br>color: %{color}<extra></extra>"
     )
+    fig = px.imshow(img_gray, binary_string=binary_string)
+    if binary_string:
+        assert fig.data[0].hovertemplate == "x: %{x}<br>y: %{y}<extra></extra>"
+    else:
+        assert (
+            fig.data[0].hovertemplate
+            == "x: %{x}<br>y: %{y}<br>color: %{z}<extra></extra>"
+        )