R help - Oct 2005 - grid.edit problem

I am having a problem in editing a grob.  It works ok if I try to
shift the grob using npc coordinates but if I do the same thing
using native coordinates the grob disappears.  What is wrong?


library(grid)
grid.newpage()

# create viewport
pushViewport(viewport(xscale = c(100,200), name = "X"))

# draw vertical line
grid.lines(150, 0:1, default.units = "native",  name = "L")

# move line 25% of the way to the right. Works ok.
grid.edit("L", x = grid.get("L")$x + unit(0.25,
"npc"))


# but now repeat it shifting it using native coordinates
########################################################

# remove line and draw a new line where the original one was
grid.remove("L")
grid.lines(150, 0:1, default.units = "native",  name = "L")

# move line 25% of the way to the right but use native coordiantes
#### line disappears !!!!!!!!!
grid.edit("L", x = grid.get("L")$x + unit(25,
"native"))

Sorry, forgot to mention my system:
> R.version.string # XP
[1] "R version 2.2.0, 2005-09-20"



On 10/15/05, Gabor Grothendieck <ggrothendieck at gmail.com>
wrote:
> I am having a problem in editing a grob.  It works ok if I try to
> shift the grob using npc coordinates but if I do the same thing
> using native coordinates the grob disappears.  What is wrong?
>
>
> library(grid)
> grid.newpage()
>
> # create viewport
> pushViewport(viewport(xscale = c(100,200), name = "X"))
>
> # draw vertical line
> grid.lines(150, 0:1, default.units = "native",  name =
"L")
>
> # move line 25% of the way to the right. Works ok.
> grid.edit("L", x = grid.get("L")$x + unit(0.25,
"npc"))
>
>
> # but now repeat it shifting it using native coordinates
> ########################################################
>
> # remove line and draw a new line where the original one was
> grid.remove("L")
> grid.lines(150, 0:1, default.units = "native",  name =
"L")
>
> # move line 25% of the way to the right but use native coordiantes
> #### line disappears !!!!!!!!!
> grid.edit("L", x = grid.get("L")$x + unit(25,
"native"))
>

Hi


Gabor Grothendieck wrote:
> I am having a problem in editing a grob.  It works ok if I try to
> shift the grob using npc coordinates but if I do the same thing
> using native coordinates the grob disappears.  What is wrong?
> 
> 
> library(grid)
> grid.newpage()
> 
> # create viewport
> pushViewport(viewport(xscale = c(100,200), name = "X"))
> 
> # draw vertical line
> grid.lines(150, 0:1, default.units = "native",  name =
"L")
> 
> # move line 25% of the way to the right. Works ok.
> grid.edit("L", x = grid.get("L")$x + unit(0.25,
"npc"))
> 
> 
> # but now repeat it shifting it using native coordinates
> ########################################################
> 
> # remove line and draw a new line where the original one was
> grid.remove("L")
> grid.lines(150, 0:1, default.units = "native",  name =
"L")
> 
> # move line 25% of the way to the right but use native coordiantes
> #### line disappears !!!!!!!!!
> grid.edit("L", x = grid.get("L")$x + unit(25,
"native"))

This is due to the fact that the *location* unit(25, "native") is very
different from the *location* unit(.25, "npc").   In your example, the
former actually corresponds to unit(-.75, "npc").

What you appear to be trying to do is add a *dimension* (width) unit(25, 
"native"), which corresponds to a *dimension* unit(.25,
"npc"), to the
original *location* unit(150, "native").  Problem is, the 'x'
component
of a  "line" is interpreted as a location so your unit(25,
"native") is
interpreted as a location.

This issue is described in one of the small grid doc's at 
http://www.stat.auckland.ac.nz/~paul/grid/doc/locndimn.pdf

A (rather verbose) way of specifying your goal is the following ...

grid.edit("L", x = grid.get("L")$x +
                    # Convert a width into a location
                    convertUnit(unit(25, "native"),
"native",
                                "x", "dimension",
"x", "location"))

... or, if you know you are only dealing with "native" (data) values, 
you could add them together before using them to specify a location ...

x <- 150 + 25
grid.edit("L", x = unit(x, "native"))

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/