def _get_total_citation_by_year(l, max_year=2015):
    """
    Calculate the total citation by year
    :param l:  list of (year, citation) tuple
    :param max_year: the maximal year
    :return: dict with the totatl number of citation in each year
    """
    min_year = int(min([y for y, v in l]))
    total_citations_dict = {}
    for i in range(min_year, int(max_year + 1)):
        total_citations_dict[str(i)] = sum([v for y, v in l if y <= i])
    return total_citations_dict