Colour Aesthetic Causes Unnecessary Blank Page in PDF File

[DarioS]

If colour is not assigned, the PDF has one page, as expected.

pdf("cars.pdf")
plot <- ggplot(mtcars, aes(drat, wt, colour = cyl)) + geom_point()
grob <- ggplotGrob(plot)
plot(1:10)
dev.off()

[hadley]

@pmur002 any thoughts on how to track this down? Presumably something in the legend code needs an active graphics device. Is there a hook I can use?

[pmur002]

I tracked this down using gdb (break on GEcurrentDevice and then track
back up the call stack).

Looks like ggplotGrob() is calling convertWidth() (at least) to
determine sizes of things and that triggers 'grid' initialisation (as
the first grid activity on the device), which triggers the new page.

But I think the bigger issue here is that the code example is a mix of
'grid' and 'graphics' drawing. This is always fraught and one general
rule if you are doing this sort of thing is always to start with a
'graphics' plot and then add 'grid' output. For example, here are some
variations that work ok ...

Remove colour aesthetic (remove legend creation)

pdf("cars-ok.pdf")
plot <- ggplot(mtcars, aes(drat, wt)) + geom_point()
grob <- ggplotGrob(plot)
plot(1:10)
dev.off()

Remove ggplotGrob()

pdf("cars-ok-too.pdf")
plot <- ggplot(mtcars, aes(drat, wt, colour = cyl)) + geom_point()
plot(1:10)
dev.off()

Put ggplotGrob() call AFTER plot() call

pdf("cars-ok-three.pdf")
plot <- ggplot(mtcars, aes(drat, wt, colour = cyl)) + geom_point()
plot(1:10)
grob <- ggplotGrob(plot)
dev.off()

Hope that helps.

Paul

On 26/01/16 05:33, Hadley Wickham wrote:

@pmur002 https://github.com/pmur002 any thoughts on how to track this
down? Presumably something in the legend code needs an active graphics
device. Is there a hook I can use?

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#1490 (comment).

Dr Paul Murrell
Department of Statistics
The University of Auckland
Private Bag 92019
Auckland
New Zealand
64 9 3737599 x85392
paul@stat.auckland.ac.nz
http://www.stat.auckland.ac.nz/~paul/

[hadley]

Thanks @pmur002! I'm going to close for now since it seems like a general problem with mixing grid and base graphics.

[DarioS]

I doubt it is caused by a mix of graphics frameworks. Consider deleting the line plot(1:10) from the example. If the colour is present, the PDF has 1 page, if not, it has 0 pages. ggplotGrob adds a new page, if the plot has colour, which I think is unintended and undocumented.