code: def top_regions_producing_zero_examples(
      df: pd.DataFrame,
  ) -> alt.Chart:
      """Creates a chart of the top regions that produced zero examples.
  
      Args:
        df: A dataframe of all regions.
  
      Returns:
        An altair chart.
      """
      regions_with_zero_examples = df[df["num examples"] == 0]
  
      runtime_of_zeros = (
          regions_with_zero_examples["total runtime"].sum() / 3600
      )
  
      total_runtime = df["total runtime"].sum() / 3600
      subtitle = (
          f"Spent {runtime_of_zeros:.2f} hours processing the "
          f"{len(regions_with_zero_examples)} regions that produced no examples, "
          f"which is {runtime_of_zeros / total_runtime * 100:.2f}% of the total "
          f"runtime of {total_runtime:.2f} hours."
      )
  
      return individual_region_bars(
          regions_with_zero_examples.nlargest(50, "total runtime"),
          title={
              "text": "The longest-running regions that produced no examples",
              "subtitle": subtitle,
          },
      )
  
imports: from typing import Dict, Sequence, List, Tuple, Text, Union
  from absl import app
  from absl import flags
  import altair
  import pandas
  import tensorflow
  from third_party.nucleus.io import sharded_file_utils
  from deepvariant import dashboard_utils
label_desc: Use image node_id 13 for calling a global function with example usage: top_regions_producing_zero_examples(df) and returns: individual_region_bars