docs: def histogram_hour_counts(
      df,
      variable
      ):
      """
      Create a day-long histogram of counts of the variable for each hour. It is
      assumed that the DataFrame index is datetime and that the variable
      `hour` exists.
      """
      if not df.index.dtype in ["datetime64[ns]", "<M8[ns]", ">M8[ns]"]:
          log.error("index is not datetime")
          return False
      counts = df.groupby(df.index.hour)[variable].count()
      counts.plot(kind = "bar", width = 1, rot = 0, alpha = 0.7)