[R] Tick mark puzzle.

[Rolf Turner]

I am bewildered by the behaviour of tickmarks as demonstrated by the
following code.  (What I'm trying to do is draw a single tick mark  
extending
from the axis all the way down to the tick label, which is two lines  
from
the axis to make sure it doesn't overlap with the ``ordinary'' tick  
labels.)

# Try 1:
# Gap between tickmark and label.
plot(1:10)
axis(side = 1, at = 2.35, labels = "2.35", tick = TRUE,
      tcl=1,line=1,col="red", col.axis = "red")
readline('Go? ')

# Try 2:
# Gap between tickmark and axis.
plot(1:10)
axis(side = 1, at = 2.35, labels = "2.35", tick = TRUE,
      tcl=-1,line=1,col="red", col.axis = "red")
readline('Go? ')

# Try 3:
# Gap between tickmark and label not filled.
plot(1:10)
axis(side = 1, at = 2.35, labels = "2.35", tick = TRUE,
tcl=1,line=1,col="red", col.axis = "red")
axis(side = 1, at = 2.35, labels ="", tick = TRUE, tcl=-1,col="red")
readline('Go? ')

# Try 4:
# Gap between tickmark and label ***is*** filled.
# Gives what I want, but ***why***?
plot(1:10)
axis(side = 1, at = 2.35, labels = "2.35", tick = TRUE,
      tcl=-1,line=1,col="red", col.axis = "red")
axis(side = 1, at = 2.35, labels ="", tick = TRUE, tcl=-1,col="red")

What I can't understand is why the tcl value in the second call to  
axis() seems
to have the opposite effect of its value in the first call to axis().

Can anyone enlighten me?

BTW setting tcl=2 and tcl=-2 have impacts which also bewilder me and  
do not
have the desired effect.

	cheers,

		Rolf Turner

P. S. Version info:

 > version
                _
platform       i386-apple-darwin8.10.1
arch           i386
os             darwin8.10.1
system         i386, darwin8.10.1
status
major          2
minor          6.0
year           2007
month          10
day            03
svn rev        43063
language       R
version.string R version 2.6.0 (2007-10-03)


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