R help - Mar 2005 - Finding the "height of a line of text" for axis

I would like to draw only the ticks of an axis, but not the axis
itself. I don't think this can be done using axis(), so I am trying to
write a cut-down version in R, which only draws ticks.

The point at which I am stuck is that the length of a tick is set by
par("tcl") as a fraction of the "height of a line of text".
So I would
like to draw a line whose length is also this length. However I cannot
find out how to convert the into units I can pass into lines(). I
thought par("cxy"), but it is too large, as is shown in the code
below. Here the red tick marks are longer than the black ones created
by axis. I have found the relevant function in plot.c:
 "GConvertYUnits(Rf_gpptr(dd)->tcl, LINES, NFC, dd);"
but can't find the corresponding R function.

Can anyone suggest how to find this out, or solve the problem in a
different way?

# Inward pointing ticks
par("tcl"=1)

drawticks <- function(at) {
 # Start of the ticks
 base<-par("usr")[1]
 # Length of the ticks
 l<-par("cxy")[2]*par("tcl")
 for (i in at) {
  lines(c(base,base+l),rep(i,2),col="red")
 }
}

# Test plot
plot(c(1,2,3), axes=FALSE)
# Draw ticks in red
drawticks(axTicks(2))
# Overprint with normal axis
axis(2, axTicks(2))

Thanks in advance,
Steven Murdoch.
-- 
w: http://www.cl.cam.ac.uk/users/sjm217/

On Wed, 2005-03-30 at 11:06 +0100, Steven J. Murdoch
wrote:
> I would like to draw only the ticks of an axis, but not the axis
> itself. I don't think this can be done using axis(), so I am trying to
> write a cut-down version in R, which only draws ticks.
> 
> The point at which I am stuck is that the length of a tick is set by
> par("tcl") as a fraction of the "height of a line of
text". So I would
> like to draw a line whose length is also this length. However I cannot
> find out how to convert the into units I can pass into lines(). I
> thought par("cxy"), but it is too large, as is shown in the code
> below. Here the red tick marks are longer than the black ones created
> by axis. I have found the relevant function in plot.c:
>  "GConvertYUnits(Rf_gpptr(dd)->tcl, LINES, NFC, dd);"
> but can't find the corresponding R function.
> 
> Can anyone suggest how to find this out, or solve the problem in a
> different way?
> 
> # Inward pointing ticks
> par("tcl"=1)
> 
> drawticks <- function(at) {
>  # Start of the ticks
>  base<-par("usr")[1]
>  # Length of the ticks
>  l<-par("cxy")[2]*par("tcl")
>  for (i in at) {
>   lines(c(base,base+l),rep(i,2),col="red")
>  }
> }
> 
> # Test plot
> plot(c(1,2,3), axes=FALSE)
> # Draw ticks in red
> drawticks(axTicks(2))
> # Overprint with normal axis
> axis(2, axTicks(2))
Steven,

Are you trying to do something like this:

 plot(1:5, axes = FALSE)
 axis(1, col.axis = "white", tcl = 1)
 axis(2, col.axis = "white", tcl = 1)

or perhaps something like this:

 plot(1:5, axes = FALSE)
 axis(1, col.axis = "white", col = "red", tcl = 1)
 axis(1, col.axis = "white", col = "black", tcl = 0)
 axis(2, col.axis = "white", col = "red", tcl = 1)
 axis(2, col.axis = "white", col = "black", tcl = 0)

or even something like this:

 plot(1:5, axes = FALSE)
 axis(1, col.axis = "white", col = "red", tcl = 1)
 axis(1, col.axis = "white", col = "white", tcl = 0)
 axis(2, col.axis = "white", col = "red", tcl = 1)
 axis(2, col.axis = "white", col = "white", tcl = 0)

If you do not want the axis labels, use:

 plot(1:5, axes = FALSE, ann = FALSE)


HTH,

Marc Schwartz