[R] placing rectangle behind plot

[Paul Murrell]

Hi


Gabor Grothendieck wrote:
> Thanks.  That's helpful.
> 
> I would be interested in the case where
> 
> 1. one does not have a variable latticeplot, as per your example,
> but just has the output of
> 
>    xyplot(x ~ x | gl(2,1), layout = 1:2)
> 
> sitting on the screen, having been "printed" by a prior
> function.  We can assume that no other graphics have been
> issued since then. Can one still create a grey rectangle behind
> the lower panel?


xyplot(x ~ x | gl(2,1), layout = 1:2)

latticeplot <- grid.grab()
# Continue as for grid.grabExpr() example ...


> 2. In fact, ideally what I would like is to create a function,
> put.in.bg, say, that works something like this:
> 
>    xyplot(x ~ x | gl(2,1), layout = 1:2)
>    trellis.focus("panel", 1, 1)
>    put.in.bg(grid.rect(w = 0.5))
>    trellis.unfocus()
> 
> or maybe
> 
>    xyplot(x ~ x | gl(2,1), layout = 1:2)
>    trellis.focus.bg("panel", 1, 1)
>    grid.rect(w = 0.5)
>    trellis.unfocus()
> 
> That allows one to add objects to a lattice panel behind the objects
> that are already there. This would also be helpful for adding grid
> lines afterwards or other lines, rectangles, etc.


I could imagine something like ...

    xyplot(x ~ x | gl(2,1), layout = 1:2)
    put.in.bg("panel", 1, 1, rectGrob(w = 0.5))

... where you just wrap the approach I described (using grid.grab() to
capture the existing plot, then modifying the resulting grob), but such
a function would obviously not work well when called after something
other than just a trellis plot.

Paul


> On 7/30/06, Paul Murrell <p.murrell at auckland.ac.nz> wrote:
>> Hi
>>
>>
>> Gabor Grothendieck wrote:
>>> I am trying to create a lattice plot and would like to later, i.e. after
>>> the plot is drawn, add a grey rectangle behind a portion of it.
>>> The following works except that the rectrangle is on top of and
>>> obscures a portion of the chart.  I also tried adding col = "transparent"
>>> to the gpar list but that did not help -- I am on windows and
>>> perhaps the windows device does not support transparency?
>>
>> Correct.
>>
>>
>>> At any rate, how can I place the rectangle behind the plotted
>>> points without drawing the rectangle first?
>>>
>>> library(lattice)
>>> library(grid)
>>> trellis.unfocus()
>>> x <- 1:10
>>> xyplot(x ~ x | gl(2,1), layout = 1:2)
>>> trellis.focus("panel", 1, 1)
>>> grid.rect(w = .5, gp = gpar(fill = "light grey"))
>>> trellis.unfocus()
>>
>> The user-interface is a little rough, but this can be done by accessing
>> the underlying grid objects.  Here's an example, with explanatory bits
>> interspersed ...
>>
>> # "grab" the lattice plot as a grid gTree
>> # There are warnings, but they are ignorable
>> latticeplot <- grid.grabExpr(print(xyplot(x ~ x | gl(2,1),
>>                                          layout = 1:2)))
>>
>> # Demonstrate that the gTree faithfully replicates the
>> # original lattice plot (not necessary, just to to what's going on)
>> grid.newpage()
>> grid.draw(latticeplot)
>>
>> # Explore the gTree (just to to show what's going on)
>> # Better user-interface would be nice here ...
>> childNames(latticeplot)
>> # Identify which children are which
>> # (appropriate grob names would be nice here)
>> lapply(latticeplot$children, class)
>> # Identify where each child is drawn
>> latticeplot$childrenvp
>> lapply(latticeplot$children, "[[", "vp")
>>
>> # Add a rect (starts off on top of everything else)
>> # NOTE that rect has to have correct vpPath
>> plotwithrect <- addGrob(latticeplot,
>>                        rectGrob(w = .5, gp = gpar(fill = "light grey"),
>>                                 vp=vpPath("plot1.toplevel.vp",
>>                                           "plot1.panel.1.1.vp")))
>>
>> # Check this draws what we expect (just to show what's going on)
>> grid.newpage()
>> grid.draw(plotwithrect)
>>
>> # Reorder children to put rect at back
>> # Appropriate user-interface would be nice here ...
>> nc <- length(plotwithrect$childrenOrder)
>> plotwithrect$childrenOrder <-
>>    plotwithrect$childrenOrder[c(nc, 1:(nc - 1))]
>>
>> # Final result
>> grid.newpage()
>> grid.draw(plotwithrect)
>>
>> 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/
>>
>>

-- 
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/