Add source example files

source/pandas/02-charts/bar-chart.py

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+import pandas as pd
+from matplotlib import pyplot
+from matplotlib.axes import Axes
+
+df = pd.read_excel("sample-data-food-sales.xlsx", sheet_name="FoodSales")
+groups = df.groupby(["Region", "City"])
+averages = groups.mean(numeric_only=True)
+
+axis: Axes = averages.plot.bar(rot=0)
+for p in axis.patches:
+    axis.annotate(f"{p.get_height():.1f}", (p.get_x() + 0.05, p.get_height() + 5), rotation=90)
+pyplot.show()