def remove_start_NaN(self, data, var=None):
        """ Remove start NaN.
        
        CHECK: Note issue with multi-column df.
        
        Parameters
        ----------
        data    : pd.DataFrame()
            Input dataframe.
        var     : list(str)
            List that specifies specific columns of dataframe.

        Returns
        -------
        pd.DataFrame()
            Dataframe starting from its first valid index.
       
        """

        # Limit to one or some variables
        if var:
            start_ok_data = data[var].first_valid_index()
        else:
            start_ok_data = data.first_valid_index()
      
        data = data.loc[start_ok_data:, :]
        return data
Remove start NaN.
        
        CHECK: Note issue with multi-column df.
        
        Parameters
        ----------
        data    : pd.DataFrame()
            Input dataframe.
        var     : list(str)
            List that specifies specific columns of dataframe.

        Returns
        -------
        pd.DataFrame()
            Dataframe starting from its first valid index.