$($Rstudio$)$

a$.$ Why is this pie chart made with a geom$\_$col$()?$ In other words, what is the difference between a stacked bar and a pie chart, from the Grammar of Graphics perspective?

b$.$ What is the line "facet$\_$grid" doing? If you're not sure, add this line on to your grouped bar

plot $($it won't be a correct plot, but you'll see the change that is made$)$