grobX for points?
On Mon, Jul 28, 2008 at 7:41 AM, Paul Murrell <p.murrell at auckland.ac.nz> wrote:
Hi Felix Andrews wrote:
Hi R-devel,
It seems that grobX and grobY (from the grid package) do not refer to
the bounding box of a grid.points object.
library(grid)
library(lattice)
xyplot(1:10 ~ 1:10)
downViewport("plot1.panel.1.1.vp")
grob <- grid.get("GRID.points", grep=TRUE)
convertX(grobWidth(grob), "cm")
# 10.3430190058480cm
# (correct, but:)
convertX(grobX(grob, "west"), "cm")
# 5.89552083333333cm
convertX(grobX(grob, "east"), "cm")
# 5.89552083333334cm
So grobX is returning the midpoint, rather than the bounding box. This
is surprising, given that ?grobX says "If the grob describes multiple
shapes, in most cases, the boundary value will correspond to the edge
of a bounding box around all of the shapes."
The grobX() and grobY() calculations for "points" grobs is "imperfect" because it treats the grob as a set of point locations and does NOT take into account the size and shape of the actual data symbols. This means that a points grob is not treated as multiple shapes, but in fact as a single set of points. Hence, the calculations of the boundary for the shape generate a convex hull around the point locations. In this case, the points all lie along a straight line, so the convex hull is a long thin "cigar" at a 45 degree angle. The calculation of points on this boundary draws a line from the centre of the convex hull to the edge. In this case, the line will intersect the convex hull boundary at the centre of the set of points for all angles (apart from possibly 45 degrees). If you want to work with the bounding box for the points, perhaps you could draw an invisible rectangle based on the range of the data and work off that, or just use convertX() on the range of the data (?)
I am doing the latter, which is easy enough. Can I suggest that grobX.Rd be updated to mention that it returns the convex hull for points, lines and segments (?) objects? It currently reads "If the grob describes multiple shapes, in most cases, the boundary value will correspond to the edge of a bounding box around all of the shapes. The exception to this is a polygon grob describing multiple polygons; in that case, the edge corresponds to a convex hull around all points of all polygons described by the grob." Regards, Felix
Paul
sessionInfo()
R version 2.7.1 (2008-06-23) i386-pc-mingw32 locale: LC_COLLATE=English_Australia.1252;LC_CTYPE=English_Australia.1252;LC_MONETARY=English_Australia.1252;LC_NUMERIC=C;LC_TIME=English_Australia.1252 attached base packages: [1] grid stats graphics grDevices utils datasets methods base other attached packages: [1] playwith_0.9-10 cairoDevice_2.8 gWidgetsRGtk2_0.0-36 RGtk2_2.12.5-3 gWidgets_0.0-28 [6] lattice_0.17-10 loaded via a namespace (and not attached): [1] gridBase_0.4-3 tools_2.7.1
-- 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/
Felix Andrews / ??? PhD candidate Integrated Catchment Assessment and Management Centre The Fenner School of Environment and Society The Australian National University (Building 48A), ACT 0200 Beijing Bag, Locked Bag 40, Kingston ACT 2604 http://www.neurofractal.org/felix/ 3358 543D AAC6 22C2 D336 80D9 360B 72DD 3E4C F5D8