def prepare_df(df):
    """
    This function converts the df header into string format
    for the gui to be able to plot

    Args:
        df --> pandas dataframe: df needs to be cleaned
    Return:
        df --> pandas dataframe: cleaned df
    """
    columns = list(df.columns.values)
    for num, item in enumerate(columns):
        if not isinstance(item, str):
            columns[num] = '{}'.format(item)
    df.columns = columns
    return df
