code: def selected_longest_and_median_regions(
      df: pd.DataFrame,
  ) -> alt.Chart:
      """Creates a stacked bar charts of the top 20 and median 20 regions.
  
      Args:
        df: A dataframe of all regions.
  
      Returns:
        An altair chart.
      """
      num_rows = len(df)
      mid = round(num_rows / 2)
  
      return individual_region_bars(
          df.iloc[0:20], "Top runtime regions"
      ) | individual_region_bars(
          df.iloc[mid - 10 : mid + 11], "Median runtime regions"
      )
  
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 12 for calling a global function with example usage: selected_longest_and_median_regions(df) and returns: unknown