Reverse plot axes with xlim=rev(range(x)) fails with asp=1

plot(y=-y,x=x, ylim=range(-y), asp=1, axes=FALSE)
box()
axis(1) 

On Sat, 5 Mar 2005, Gregory Jefferis wrote:

Dear R users,

I would like to reverse the axes on some xy plots (for example to set the
origin at the top left rather than the bottom left).  I had planned to use
something of the following form:

plot(y=y<-c(20,4,5,6),x=x<-c(10,20,30,40),ylim=rev(range(y)))

ie reversing ylim to reverse the y axis.  This works fine however I also
want to use the parameter asp=1 to ensure that equal distances in the x or y
direction are plotted as such on the screen.  However the simple example I
showed above now fails:

plot(y=y<-c(20,4,5,6),x=x<-c(10,20,30,40),ylim=rev(range(y)),asp=1)

The actual ylim used is considerably less than it should be, so no points
appear on the plot.

Can anyone suggest a simple remedy?

plot(y=-y,x=x, ylim=range(-y), asp=1, axes=FALSE)
box()
axis(1)
axis(2, at=seq(-20,-5,5), label=seq(20,5,-5))

is a simple kludge, the explanation is touched on in a comment in
src/main/plot.c, after line 564:

 * The use of asp can have weird effects when axis is an
 * interpreted function.  It has to be internal so that the
 * full computation is captured in the display list.

and may be related to the sign of ydelta being dropped in line 652 (for
the y axis) and Gscale() being possibly called in line 666 with incorrect
assumptions about whether to add or subtract the correction factor -
untried!

Many thanks,

Greg Jefferis.

PS I would guess that the origin of this behaviour is that plot.window
expects xlim (and ylim) to be in the order xmin,xmax (or ymin,ymax) when it
sets about redefining xlim and ylim if asp!=NA.  If that suggestion is
correct, I'm not quite sure why the restriction is necessary.