fix: density was not working with special storages (#134)

docs/changelog.rst

@@ -1,6 +1,14 @@
 Changelog
 ====================
 
+
+Version 2.1.1
+--------------------
+
+* Fix density (and density based previews)
+  `#134 <https://github.com/scikit-hep/hist/pull/134>`_
+
+
 Version 2.1.0
 --------------------
 
@@ -26,7 +34,7 @@ Version 2.0.1
 * Fixed ``plot2d_full`` incorrectly mirroring the y-axis.
   `#105 <https://github.com/scikit-hep/hist/pull/105>`_
 
-* `Hist.plot_pull`: more suitable bands in the pull bands 1sigma, 2 sigma, etc.
+* ``Hist.plot_pull``: more suitable bands in the pull bands 1sigma, 2 sigma, etc.
   `#102 <https://github.com/scikit-hep/hist/pull/102>`_
 
 * Fixed classichist's usage of `get_terminal_size` to support not running in a terminal

src/hist/basehist.py

@@ -214,7 +214,7 @@ def density(self) -> np.ndarray:
         """
         Density numpy array.
         """
-        total = self.sum() * functools.reduce(operator.mul, self.axes.widths)
+        total = np.sum(self.values()) * functools.reduce(operator.mul, self.axes.widths)
         return self.values() / np.where(total > 0, total, 1)
 
     def show(self, **kwargs):

tests/test_general.py

@@ -759,6 +759,14 @@ def test_general_density():
         assert pytest.approx(sum(h.density()), 2) == pytest.approx(10 / 6, 2)
 
 
+def test_weighted_density():
+    for data in range(10, 20, 10):
+        h = Hist(axis.Regular(10, -3, 3, name="x"), storage="weight").fill(
+            np.random.randn(data)
+        )
+        assert pytest.approx(sum(h.density()), 2) == pytest.approx(10 / 6, 2)
+
+
 def test_general_axestuple():
     """
     Test general axes tuple -- whether Hist axes tuple work properly.