l1: {1900, 1910, 1920, 1930, 1940, 1950, 1960, 1970, 1980, 1990, 2000}
l2: {2.7, 2.9, 3.1, 3.5, 3.7, 4, 4.4, 4.6, 4.8, 5, 5.2}
a = BarChart(l1, l2, 0.3)