adding histogram example

ch3/bar_histogram.py

@@ -0,0 +1,37 @@
+"""
+Histogram: a diagram consisting of rectangles whose area is proportional to the frequency
+of a variable and whose width is equal to the class interval.
+
+"""
+
+from collections import Counter
+from matplotlib import pyplot as plt
+
+grades = [83, 95, 91, 87, 70, 0, 85, 82, 100, 67, 73, 77, 0]
+
+
+def decile(grade: int) -> int:
+    """Returns the decile the grade falls into.
+
+    Floor division must be used.
+    """
+
+    return grade // 10 * 10
+
+
+histogram = Counter(decile(grade) for grade in grades)
+
+# Note We do not need to use the alignment tricks to center our bars
+# That was a hold over from the previous Matplob lib API
+plt.bar(x=histogram.keys(), height=histogram.values(), width=8)
+
+# Used to set the axis scale x-min, x-max, y-min, y-max
+# We want to fully show the x axis
+# You usually always want your y to start at 0
+plt.axis([-5, 105, 0, 5])
+
+plt.xticks(range(0, 110, 10))
+plt.xlabel("Decile")
+plt.ylabel("# of Students")
+plt.title("Distribution of Exam 1 Grades")
+plt.show()