[R] Problem going back to a viewport with gridBase

[Paul Murrell]

Hi


Gabor Grothendieck wrote:
> I am setting up base plots -- one in viewport A and and one in B.  This part
> works fine.  But if I go back to A after having done B and add
> horizontal lines it seems
> to not use the correct coordinates.  How do I tell it to resume using A's
> coordinates?  I am already using par(fig = gridFIG()) but it seems that that's
> not enough to reestablish them.  What happens is that when I go back to 
> A it draws the horizontal lines as if its relative to B's coordinates
> rather than
> restablishing A's coordinates.  As a result the horizontal lines are
> drawn near the
> bottom of the graph instead of at the correct heights.  Try running the code
> below to see what I mean.
> 
> I have also tried to use baseViewports with this but did not have any
> success.
> 
> How do I modify this example so that the horizontal red lines come out
> at the appropriate levels?    Note that this is just an example and in 
> the future I will want to have multiple viewports each with a base plot and
> add arbitrary additional line or point plots to them so the solution needs
> to be sufficiently general that I can so generalize it.
> 
> Thanks.
> 
> 
> library(gridBase)
> 
> opar <- par(no.readonly = TRUE)
> grid.newpage()
> 
> # two columns, one row
> unit. <- unit(c(1,1), c("null","null"))
> pushViewport(viewport(layout = grid.layout(1, 2, widths = unit.)))
> 
> # draw green graph in first column (viewport A)
> pushViewport(viewport(layout.pos.col = 1, name = "A"))
> par(fig = gridFIG()); par(new = TRUE)
> plot(1:10, col = "green", pch = 20)
> upViewport(1)
> 
> # draw purple graph in second column (viewport B)
> pushViewport(viewport(layout.pos.col = 2, name = "B"))
> par(fig = gridFIG()); par(new = TRUE)
> plot(1:100, col = "purple", pch = 18)
> upViewport()
> 
> # go back to A and add horizontal grid lines
> seekViewport("A")
> par(fig = gridFIG())
> abline(h=1:10, col = "red")  #### THESE DO NOT GET DRAWN AS EXPECTED
> popViewport()
> 
> # go back to B and add vertical grid lines
> seekViewport("B")
> par(fig = gridFIG())
> abline(v=1:10, col = "red")
> popViewport()
> par(opar)


The base, or "traditional", graphics system only records the *current* 
plotting coordinates;  it does not retain a memory of previous plotting 
coordinates.  What your example does is *reposition* the plotting 
region, but to do what you want you would have to recreate the plotting 
coordinates of the first plot.  This is possible (at least in simple 
cases like the above), as shown below.  However, perhaps a better 
approach would be to use a combination of grid and lattice plots, where 
the coordinate systems are retained and don't need to be recreated.  An 
example of this approach is given at the end.

#######
# Modified example using gridBase
#######
library(gridBase)

opar <- par(no.readonly = TRUE)
grid.newpage()

# two columns, one row
unit. <- unit(c(1,1), c("null","null"))
pushViewport(viewport(layout = grid.layout(1, 2, widths = unit.)))

# draw green graph in first column (viewport A)
pushViewport(viewport(layout.pos.col = 1, name = "A"))
par(fig = gridFIG()); par(new = TRUE)
plot(1:10, col = "green", pch = 20)
upViewport(1)

# draw purple graph in second column (viewport B)
pushViewport(viewport(layout.pos.col = 2, name = "B"))
par(fig = gridFIG()); par(new = TRUE)
plot(1:100, col = "purple", pch = 18)
upViewport()

# go back to A and add horizontal grid lines
seekViewport("A")
par(fig = gridFIG()); par(new=TRUE)  #### extra par(new=TRUE)
plot(1:10, type="n", axes=FALSE, ann=FALSE)  #### RESET PLOT A AXES
abline(h=1:10, col = "red")
popViewport()

# go back to B and add vertical grid lines
seekViewport("B")
par(fig = gridFIG()); par(new=TRUE)  #### extra par(new=TRUE)
plot(1:100, type="n", axes=FALSE, ann=FALSE)  #### RESET PLOT B AXES
abline(v=1:10, col = "red")
popViewport()
par(opar)


#######
# Similar result but using grid and lattice
#######
library(grid)
library(lattice)

grid.newpage()

# two columns, one row
unit. <- unit(c(1,1), c("null","null"))
pushViewport(viewport(layout = grid.layout(1, 2, widths = unit.)))

# draw green graph in first column (viewport A)
pushViewport(viewport(layout.pos.col = 1, name = "A"))
# lattice plot instead of base plot
p1 <- xyplot(1:10 ~ 1:10, col="green", pch=20)
# prefix important so I can refer to it later
print(p1, newpage=FALSE, prefix="plotA")
upViewport(1)

# draw purple graph in second column (viewport B)
pushViewport(viewport(layout.pos.col = 2, name = "B"))
p2 <- xyplot(1:100 ~ 1:100, col="purple", pch=18)
print(p2, newpage=FALSE, prefix="plotB")
upViewport()

# go back to A and add horizontal grid lines
seekViewport(trellis.vpname("panel", 1, 1, prefix="plotA"))
# I'm working on a grid.abline() ...
grid.segments(x0=0, x1=1,
               y0=unit(1:10, "native"),
               y1=unit(1:10, "native"),
               gp=gpar(col="red"))

# go back to B and add vertical grid lines
seekViewport(trellis.vpname("panel", 1, 1, prefix="plotB"))
grid.segments(y0=0, y1=1,
               x0=unit(1:10, "native"),
               x1=unit(1:10, "native"),
               gp=gpar(col="red"))

upViewport(0)

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/