fix(guides): don't evaluate positional after_scale modifiers for legends

Legend key data can only contain the matched legend aesthetics and the
geom's default aesthetics. Staged positional aesthetics (e.g. x, y)
never appear in it, so evaluating their after_scale expressions against
the key data raised a PlotnineError, triggered a spurious "this should
not happen" warning, and discarded all legitimate legend modifiers via
the fallback path. Filter the modifiers to aesthetics that can be in
the key data before evaluation.

closes #1067

Author: has2k1

plotnine/guides/guide_legend.py

@@ -184,10 +184,11 @@ def create_geoms(self):
             # Modify aesthetics
 
             # When doing after_scale evaluations, we only consider those
-            # for the aesthetics that are valid for this layer/geom.
+            # for aesthetics that can be in the legend key data.
+            legend_aes = l.geom.DEFAULT_AES.keys() | matched_set
             aes_modifiers = {
                 ae: l.mapping._scaled[ae]
-                for ae in l.geom.aesthetics() & l.mapping._scaled.keys()
+                for ae in legend_aes & l.mapping._scaled.keys()
             }
 
             try:

tests/test_guide_internals.py

@@ -8,8 +8,9 @@
     geom_bar,
     geom_point,
     ggplot,
+    stage,
 )
-from plotnine.data import mtcars
+from plotnine.data import mpg, mtcars
 
 
 def test_no_after_scale_warning():
@@ -20,6 +21,18 @@ def test_no_after_scale_warning():
         p.draw_test()  # type: ignore
 
 
+def test_after_scale_positional_aesthetic_with_legend():
+    # A staged positional aesthetic cannot appear in the legend key
+    # data, so building the legend must not attempt to evaluate it
+    p = ggplot(mpg, aes("drv", "displ", color="drv")) + geom_point(
+        aes(x=stage("drv", after_scale="x"))
+    )
+
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        p.draw_test()  # pyright: ignore[reportAttributeAccessIssue]
+
+
 def test_guide_legend_after_scale():
     def alphen(series, a):
         ha = f"{round(a * 255):#04X}"[2:]