fix: plot1d overlay to work for IntCat axis corrrectly (#419)

Reported by @DryRun

---------

Signed-off-by: Henry Schreiner <[email protected]>
Co-authored-by: Henry Schreiner <[email protected]>

Author: andrzejnovak

docs/changelog.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## WIP
+
+* Support IntCat for plot1d overlay
+  [#419](https://github.com/scikit-hep/hist/pull/419)
+
 ## Version 2.7.3
 
 This release fixes an issue with Python 3.12 pulling in NumPy 2 pre-releases.

src/hist/basehist.py

@@ -502,8 +502,9 @@ def plot1d(
             (overlay,) = (i for i, ax in enumerate(self.axes) if ax.traits.discrete)
         assert overlay is not None
         cat_ax = self.axes[overlay]
-        cats = cat_ax if cat_ax.traits.discrete else np.arange(len(cat_ax.centers))
-        d1hists = [self[{overlay: cat}] for cat in cats]
+        icats = np.arange(len(cat_ax))
+        cats = cat_ax if cat_ax.traits.discrete else icats
+        d1hists = [self[{overlay: cat}] for cat in icats]
         if "label" in kwargs:
             if not isinstance(kwargs["label"], str) and len(kwargs["label"]) == len(
                 cats