Polish waterfall implementation per review

Agent-Logs-Url: https://github.com/sdpython/teachpyx/sessions/e6e8e7c7-c0a1-47b3-9763-5ebb04affcb9

Co-authored-by: xadupre <22452781+xadupre@users.noreply.github.com>

Author: Copilot

_unittests/ut_tools/test_pandas.py

@@ -1,4 +1,5 @@
 import unittest
+from matplotlib.axes import Axes
 import pandas
 from teachpyx.ext_test_case import ExtTestCase
 from teachpyx.tools.pandas import plot_waterfall, read_csv_cached
@@ -23,7 +24,7 @@ def test_plot_waterfall(self):
             }
         )
         ax, plot_df = plot_waterfall(df, "delta", "name", total_label="TOTAL")
-        self.assertEqual(ax.__class__.__name__, "Axes")
+        self.assertIsInstance(ax, Axes)
         self.assertEqual(list(plot_df["label"]), ["A", "B", "C", "TOTAL"])
         self.assertEqual(list(plot_df["start"]), [0.0, 10.0, 7.0, 0.0])
         self.assertEqual(list(plot_df["end"]), [10.0, 7.0, 12.0, 12.0])

teachpyx/tools/pandas.py

@@ -97,7 +97,7 @@ def plot_waterfall(
         }
     )
 
-    total = float(values.sum()) if len(values) > 0 else 0.0
+    total = float(values.sum())
     total_row = pandas.DataFrame(
         {
             "label": [total_label],