Hi
Steven J. Murdoch wrote:
I would like to draw a vertical line from a given point, in user coordinates, to x inches before from another point, also in user coordinates. This is easy enough to do for linear scales, using code based on xinch/yinch, but I do not know how to do this for logarithmic scales.
This code shows an example of what I mean[1]:
split.screen(c(2,1)) screen(1) # Linear scale, works fine plot(1:100, cex=0.5, pch=19) plotheight <- diff(par("usr")[3:4]) igap = 0.5 # intended gap, in inches gap <- igap/par("pin")[2]*plotheight # gap in user units lines(c(20,20),c(100,60+gap)) lines(c(20,20),c(60-gap,1)) points(20,60, pch=18)
screen(2) # Logarithmic scale, point no longer centered plot(1:100, cex=0.5, pch=19, log="y") plotheight <- diff(par("usr")[3:4]) igap = 10 gap <- igap/par("pin")[2]*plotheight lines(c(20,20),c(100,10+gap)) lines(c(20,20),c(10-gap,1)) points(20,10, pch=18) close.screen(all=TRUE)
The top graph is as I want. The diamond is centered in the gap, and the gap is 1 inch high (2*igap).
In bottom graph, using a log scale, the diamond is no longer centered in the gap and there is a non-linear relationship between the gap height in inches and igap.
I understand why this is happening, and this is why the xinch and yinch functions raise a warning, but is there a way to handle it? For example, is there a function which will return the user coordinate of a point, x inches above a given point p (in user coordinates), for a logarithmic y axis?
No function, but does this do what you want? The basic idea is to work in logged values (which is what par("usr") is in) then convert back to plot (broken into many steps to hopefully aid clarity) ...
plot(1:100, cex=0.5, pch=19, log="y")
plotheight <- diff(par("usr")[3:4])
igap <- 0.5 # intended gap, in inches
pigap <- igap/par("pin")[2] # intended gap as propn of plot height
dy <- 10 # y location for diamond
ldy <- log10(dy) # logged y location for diamond
upper <- ldy + pigap/2*plotheight # logged y location for top of gap
lower <- ldy - pigap/2*plotheight # logged y location for bottom of gap
lines(c(20,20),c(100, 10^upper))
lines(c(20,20),c(10^lower, 1))
points(20,10, pch=18)
Paul
[1] I actually want to use this in a larger script, where I leave a gap in the axis where the median in. This needs to be small but legible to the eye, so that is why I am defining it in inches. The source code is at: http://www.cl.cam.ac.uk/users/sjm217/projects/graphics/fancyaxis.R
-- Dr Paul Murrell Department of Statistics The University of Auckland Private Bag 92019 Auckland New Zealand 64 9 3737599 x85392 [EMAIL PROTECTED] http://www.stat.auckland.ac.nz/~paul/
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