[R] confused about x-coordinates and bar charts

[Marc Schwartz]

On Fri, 2003-07-18 at 13:41, Jay Pfaffman wrote:
> Thanks for all the help on my previous histogram problem.  I intend to
> summarize the solutions back to the list Real Soon Now, but first, I've
> got another problem.
> 
> I've made a bar chart that reports means.  I'd like to put the number
> of observations on top of each bar.  Here's what I've got:
> 
> barplot((subset$x),
>         col=grey(.5),
>         ylab="Mean Engagement Rating",
>         xlab="Comment Category",
>         main="All Engagement Ratings",
>         ylim=c(0,7),
>         cex.names=.75,
>         names.arg=mynames
>         )
> 
> 
> for (i in 1:7){
>   text(i,as.numeric(subset$x[i])+.5,counts$x[i])
> }
> 
> 
> This would seem to do the trick, but the numbers are not centered over
> the bars.  They're a bit to the right on the bars on the left and
> almost right on the bars on the right.  I don't understand why the
> scale is not the same.  
> 
> Thanks again for your help.


You need to get the bar midpoints (which are not integer values) and are
returned by barplot() 'silently'. You must assign these to a separate
variable to be used when calling barplot().  

Thus, change your code to:

mp <- barplot(subset$x,
        col=grey(.5),
        ylab="Mean Engagement Rating",
        xlab="Comment Category",
        main="All Engagement Ratings",
        ylim=c(0,7),
        cex.names=.75,
        names.arg=mynames
        )

Now to place the bar values above the bar tops use: 

text(mp, subset$x, labels = subset$x, pos = 3)

The 'pos = 3' argument, will place the text above the top of the bars. 

Also, note that you do not need a for loop here. Remember, R is vector
based. A single call to text() will cycle through the values.

See ?barplot and ?text for more information.

HTH,

Marc Schwartz