Problem going back to a viewport with gridBase

On 5/31/05, Paul Murrell <p.murrell at auckland.ac.nz> wrote:

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/

Thanks.  I will study that further.  One other question:

Using layout or mfcol/mfrow (both from base graphics) one can 
set it up so each new plot goes into a successive cell.  That is one can
do a traversal of the cells in a layout by just issuing successive
calls to plot.  Is there something analogous to that in grid?   What I
am doing right now is to calculate the row and column of the next cell
and then move to it like this:

   # mm.row[j] gives the row in the layout of the jth cell
   # mm.col[j] gives the col in the layout of the jth cell
   mm <- matrix(seq(nr*nc), nr, nc)
   mm.row <- c(row(mm))
   mm.col <- c(col(mm))

  # go to next cell in the array
   j <- j + 1 # increment position
  pushViewport(viewport(layout.pos.row = mm.row[j], layout.pos.col = mm.col[j]))

Is that how to do it or is there some layout/mfcol-like way?