Can't increase title and x/y label size in a ggplot2 plot saved as a PNG file, but it works fine on screen - r

Question

I am hitting a small, but not insignificant brick wall with this oft asked and answered question.
I am using Rstudio 0.97.336 and R 3.0.0 on Linux. I am making a (much more complex) graph to put in a paper. The default size of the title and x/y labels are too small to be easily read. However the obvious method for fixing this using the theme function on element_text
theme(axis.title.y = element_text(size = rel(1.8))
does not work, if I save the image as a PNG file. It does however work, exactly as expected, when I'm looking at the images in RStudio. The code below reproduces my problem exactly.
##Libraries
library(ggplot2)
set.seed(15612)
##Generate data
Year <- seq(2000,2010)
data <- -2*(Year - 2005) + 10 + runif(11,min=-3,max=3)
Title <- "Title for our graph"
xlab <- "X label"
ylab <- "Y label"
df <- data.frame(Year,data)
##Plot
##First image with small title, xlab, ylab
image1 <- ggplot(df) +
geom_line(aes(x=Year,y=data)) +
theme_bw() +
labs(title=Title,xlab=xlab,ylab=ylab)+
theme(panel.border = element_rect(fill = NA, colour="grey70"))
image1
ggsave("Image1.png",image1, width=15,height=10,units='cm')
##Second image with larger title, xlab, ylab
image2 <- image1 +
theme(axis.title.y = element_text(size = rel(1.8), angle = 90)) +
theme(axis.title.x = element_text(size = rel(1.8), angle = 00)) +
theme(plot.title = element_text(size = rel(2.0), angle = 00))
image2
ggsave("Image2.png",image2, width=15,height=10,units='cm')
dev.off()
image1
image2
These images look exactly as expected on the screen in Rstudio. Image 1 has small font sizes for the title, etc. and image 2 has larger more legible font sizes. Unfortunately, when saved as png files, they are identical, and both have small fonts for the title, x and y labels.
I can't (yet) post images, so if you look at these two urls, you will see the problem.
Image 1 - small title font
Image 2 - still a small title font, but ought to be bigger
I cannot see where I am going astray. I know there are issues (or features!) with lazy evaluation in ggplot2, but I don't see where this is biting me. I would be very grateful for any help with this,
Regards,
Anthony Staines

Answer 1

Using RStudio, I am also seeing some strange behaviour (but I need to look into the docs a bit more to decide if it is not as we should expect), however, I think you can get the output you expect by calling ggsave, letting it use it's default plot = last.plot(), then running the plot then calling dev.off() between the plots. i.e.
The workaround
ggsave("~/Image1.png", width=15,height=10,units='cm')
image1
dev.off()
ggsave("~/Image2.png", width=15,height=10,units='cm')
image2
dev.off()
A reproducible example of this behaviour
If we try the following example in RStudio I can get the same behaviour as the OP. Running the first code block below in RGui 3.0.0 gives us what we expect, i.e. the 3rd picture. However this is what happens in RStudio:
## Make plot and save
qp <- qplot(1:5, rnorm(5), size = I(2) )
qp
ggsave("~/Image1.png", width=15,height=10,units='cm')
## Make new plot
qp <- qplot(1:10, rnorm(10), size = I(5) )
qp
ggsave("~/Image2.png", width=15,height=10,units='cm')
At this point if we try to open the files that are saved we get:
Then we just run dev.off()
## Without calling dev.off() plot 1 is still open and displays nothing
## Plot two is accessible from the filesystem
## Calling dev.off() we then get both plots, but BOTH plots
## use settings from plot 2
dev.off()
And we get:
Now if we try and save the plots by calling ggsave then printing the plots to screen and then calling dev.off() it works as expected:
## Now we try calling dev.off() between plots:
qp <- qplot(1:5, rnorm(5), size = I(2) )
ggsave("~/Image1.png", width=15,height=10,units='cm')
qp
dev.off()
## Make new plot
qp <- qplot(1:10, rnorm(10), size = I(5))
ggsave("~/Image2.png", width=15,height=10,units='cm')
qp
dev.off()
We then get: