It happens more than often that a packages implements a plotting function of itself (often routed as a convenience for exploratory data analysis, though in my opinion, having access to underlying data directly would be way more useful) for quick visualization.
However, what if you don't like the way it's plotted? Maybe you need to change
the alpha values, or assign different labels or colors? Adding new geoms to
the ggplot object does not work.
Today I learned that we can deconstruct the ggplot2 object into its plot
constituents and modify them. For example, consider you can only access p and
want to add colors.
p <- ggplot(iris) + geom_point(aes(Sepal.Width, Petal.Width))
p
In that case,
obj_p <- ggplot_build(p)
obj_p$data[[1]]$colour <- 'red'
new_p <- ggplot_gtable(obj_p)
plot(new_p)
The new_p should have the points colored red.