Exercise 3.1

ggplot(nass.cotton, aes(year, yield)) + 
  geom_point(aes(color = acres)) + 
  facet_wrap(~state) + 
  scale_y_log10(name = "Yield") + 
  scale_color_continuous_sequential(palette = "ag_GrnYl",
                                    trans = "log10",
                                    breaks = trans_breaks("log10", function(x) 10^x),
                                    labels = trans_format("log10", math_format(10.^.x))) +
  labs(x = "Year", title = "US Cotton Yields", subtitle = "1900 to 2011")