+= file evaluation prints a warning if zero-rows are dropped

        df.drop(_extract_stamps_by_src_and_kind(stamps, "sender"), axis=1, inplace=True)

    # Drop rows with value 0 (as they have probably not been written out).

    return df[df[measured_column + "_T"] != 0]

    res = df[df[measured_column + "_T"] != 0]

    total, filtered = df.shape[0], res.shape[0]

    if filtered < total:

        print("{}: ignored {} of {} packets due to missing timestamps.".format(file_name, total-filtered, total))

    return res

def _diff_t_c(df, start, stop):