[R] barplot(names.arg) versus axis(labels)

[Marc Schwartz]

On Fri, 2003-11-07 at 14:29, Siddique, Amer wrote:
> Should I be able to use axis() on a barplot?  i have a data.frame, the first
> 3 values of which are:
> 
> > c[1:3,]
>     median      mean
> A1    56.5  58.50000
> A61   73.0  73.00000
> A62   63.0  63.00000
> 
> > str(c)
> `data.frame':   19 obs. of  2 variables:
>  $ median: num  56.5 73 63 161 51 55 44.5 22 54 49 ...
>  $ mean  : num   58.5  73.0  63.0 161.0  47.5 ...
> 
> if I do barplot(median) and then try to label the bars with axis(), I get;
> 
> > axis(1,,labels=rownames(c),font=4,cex=1)
          ^^
You have an error in your call to axis().

You are missing the 'at' argument. As a result, the default values of
'at' are set to axTicks(1), which in this case is returning 5 values.
You have passed 19 values to the 'labels' argument in axis(). Hence the
error message below.

> Error in axis(side, at, labels, tick, line, pos, outer, font, vfont, lty,  :
> 
>         location and label lengths differ, 5 != 19
> 
> even though 
> 
> barplot(median, names.arg=rownames(c)) works and
> 
> > length(rownames(c))
> [1] 19


In the above call to barplot(), you have specified the bar names, which
will be matched within the function to the number of bars, thus it
works.

> also when I attempt to place the value of the observation
>  above the bar it does not space properly across all bars:
> 
> text(median,labels=median,pos=3,cex=0.6)
> 
> do i need to explicitly state an x-pos for the co-ord argument here?

Yes. You get the proper x axis values by calling barplot() in the
following fashion:

mp <- barplot(....)

In this case, barplot() returns the bar midpoints and assigns them to
'mp'.

Once you have that information, you can call text() with the x values
set to 'mp'.

See ?barplot for examples and you may wish to review the most recent R
News, where there is an article on R's base graphics in R Help Desk.

http://cran.r-project.org/doc/Rnews/Rnews_2003-2.pdf

Just beware though....the author of that article has been known indulge
in single malt scotches on Friday afternoons...  ;-)

HTH,

Marc Schwartz