Exercise 9. Dollars per day - part 3 - multiple density plots

Now we are going to combine the plotting tools we have used in the past two exercises to create density plots for multiple years.

100 XP

Create the dollars_per_day variable as in Exercise 7, but for African countries in the years 1970 and 2010 this time.
- Make sure you remove any NA values.
Create a smooth density plot of dollars per day for 1970 and 2010 using a log (base 2) scale for the x axis.
Use facet_grid to show a different density plot for 1970 and 2010.