data set includes: country, year, participant in wwi (1 if yes, 0 if no), and toprate (highest marginal income tax rate, In r: Create a line plot of the mean of the highest marginal income tax rate in each year for countries that participated in World War I at some point (i.e., participant = 1). Add another line to the plot of the mean of the highest marginal income tax rate for countries that did not participate in WWI