class_docstr: Force Index (FI)
  
  It illustrates how strong the actual buying or selling pressure is. High
  positive values mean there is a strong rising trend, and low values signify
  a strong downward trend.
  
  http://stockcharts.com/school/doku.php?id=chart_school:technical_indicators:force_index
  
  Args:
      close(pandas.Series): dataset 'Close' column.
      volume(pandas.Series): dataset 'Volume' column.
      window(int): n period.
      fillna(bool): if True, fill nan values.
docstr: Force Index (FI)
  
  Returns:
      pandas.Series: New feature generated.
docstr_tok: ["Force","Index","(","FI",")","Returns",":","pandas.Series",":","New","feature","generated","."]
code: def force_index(self) -> pd.Series:
      """Force Index (FI)
  
      Returns:
          pandas.Series: New feature generated.
      """
      fi_series = self._check_fillna(self._fi, value=0)
      return pd.Series(fi_series, name=f"fi_{self._window}")
  
imports: import typing
  import numpy
  import pandas
  from ta.utils import IndicatorMixin, _ema
label_desc: Use image node_id 3 for calling the ForceIndexIndicator obj's underlying member method code with example usage: obj.force_index() and returns: pd