Adding lines to barplots + 2 y-axes

Explanation as to why base plot should be saved as a separate object and referenced in subsequent plots: https://www.r-bloggers.com/adding-lines-or-points-to-an-existing-barplot/

Good code example: https://stackoverflow.com/questions/15277712/adding-lines-to-barplot-in-a-different-y-axis-in-r