def DiffColumns(col1, col2):
    """Computes the difference between two columns.

    col1: map from int year to float datum
    col2: map from int year to float datum

    Returns: map from int year to float difference
    """
    years1 = set(col1)
    years2 = set(col2)
    res = [(year, col1[year] - col2[year])for year in sorted(years1 & years2)]
    return zip(*res)