modifying ggplot objects after creation

Question

Is there a preferred way to modify ggplot objects after creation?

For example I recommend my students to save the r object together with the pdf file for later changes...

library(ggplot2) graph <-    ggplot(mtcars, aes(x=mpg, y=qsec, fill=cyl)) +   geom_point() +   geom_text(aes(label=rownames(mtcars))) +   xlab('miles per galon') +   ggtitle('my title')  ggsave('test.pdf', graph) save(graph, file='graph.RData') 

So new, in case they have to change title or labels or sometimes other things, they can easily load the object and change simple things.

load('graph.RData') print(graph) graph +    ggtitle('better title') +   ylab('seconds per quarter mile') 

What do I have to do for example to change the colour to discrete scale? In the original plot I would wrap the y in as.factor. But is there a way to do it afterwards? Or is there a better way on modifying the objects, when the data is gone. Would love to get some advice.

Answer 1

You could use ggplot_build() to alter the plot without the code or data:

Example plot:

data("iris")  p <- ggplot(iris) +    aes(x = Sepal.Length, y = Sepal.Width, colour = Species) +    geom_point() 

Colours are respective to Species.

Disassemble the plot using ggplot_build():

q <- ggplot_build(p) 

Take a look at the object q to see what is happening here. To change the colour of the point, you can alter the respective table in q:

q$data[[1]]$colour <- "black" 

Reassemble the plot using ggplot_gtable():

q <- ggplot_gtable(q) 

And plot it:

plot(q) 

Now, the points are black.