This example demonstrates how to verify a function that filters histogram data using Python's `collections.Counter`. It shows a `threshold_above` function returning keys whose values exceed a given threshold, as confirmed by a doctest embedded in the docstring. The code imports `collections` and defines the function with a doctest example, illustrating the expected output for a sample counter. The visualization relies on Python’s standard library, specifically `collections.Counter` and doctest, to validate behavior without external plotting libraries. The example highlights testing practices for data-processing logic, not graphical rendering.