[R] Mixing grid and base graphics--need help understanding this quirk

[Paul Murrell]

Hi


Kevin Wright wrote:
> My setup: Windows 2000, R 2.3.1
> 
> When I start a brand new session of R and paste the code below into R,
> the graphic device shows "Some text" in the lower left corner.  If I
> paste the code into the command window again, then "Some text" does
> not appear in the lower left corner.  Why is this?
> 
> require(grid)
> par(mfrow=c(1,2))
> plot(1:10)
> plot(-10:1)
> par(mfrow=c(1,1))
> pushViewport(viewport(.04, .04, width=stringWidth("Some text"),
> height=unit(2,"lines"),
>                       name="pagenum", gp=gpar(fontsize=10)))
> grid.text("Some text", gp=gpar(col="gray30"),
>           just=c("left","bottom"))
> popViewport()


Good question! :)

Short answer:
Traditional and grid graphics are fighting each other for control of the
clipping region.

Simple fix/workaround:
Explicitly set the clipping region for the grid output, like this ...

require(grid)
par(mfrow=c(1,2))
plot(1:10)
plot(-10:1)
par(mfrow=c(1,1))
# Requires R >= 2.3.0
grid.clip()
pushViewport(viewport(.04, .04, width=stringWidth("Some text"),
height=unit(2,"lines"),
                      name="pagenum", gp=gpar(fontsize=10)))
grid.text("Some text", gp=gpar(col="gray30"),
          just=c("left","bottom"))
popViewport()

Long answer:
The first time you run the code, the pushViewport() call is the first
grid operation on the graphics device, so some initial grid set up
occurs, *including setting the clipping region to the whole device*.
The second time you run the code, the pushViewport() call just inherits
the current clipping region (which is the one set by the last
traditional plot) so the text gets clipped.  You can see this effect,
with the following code (the text does not even appear the first time
because the grid initialisation occurs before the traditional plots) ...

require(grid)
grid.newpage()
par(mfrow=c(1,2))
plot(1:10)
plot(-10:1)
par(mfrow=c(1,1))
pushViewport(viewport(.04, .04, width=stringWidth("Some text"),
height=unit(2,"lines"),
                      name="pagenum", gp=gpar(fontsize=10)))
grid.text("Some text", gp=gpar(col="gray30"),
          just=c("left","bottom"))
popViewport()

... so really the problem is that the text should never appear.

Interestingly, if you run your original code (text is drawn), then run
it again (text is not drawn), then ONLY run the grid part (push, text,
pop) the text DOES get drawn!  This happens because the popViewport()
restores the clipping region of the previous grid viewport (which is the
whole device).

I need to think a bit more about whether this behaviour can be "fixed",
whether it should be fixed, or whether the current "user beware" warning
about mixing traditional and grid graphics is sufficient.

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