[R] Using factor levels as coordinates with geom_rect

[Loris Bennett]

Hi,

With ggplot2 I can use the following to create a rectangle 

geom_rect(aes(ymin=as.Date("8-Apr-2014", format="%d-%b-%Y"),
                ymax=as.Date("30-Apr-2014", format="%d-%b-%Y"),
                xmin="node002",xmax="node098"),

where the x values are levels of a factor.  This works if I want the
rectangle to extend across a range of factor level.  My question is this:

How can I create a similar rectangle around a single factor level?

My assumption is that I should be able to convert the factor to a
numerical value.  I could then subtract and add a smaller number to
obtain xmin and xmax, respectively.  However, I don't know how to
convert the factor level to a value which corresponds to its
x-coordinate on the plot.

I posted a longer version of this question on StackOverflow with the
full code, input data, and a plot of the output:

http://stackoverflow.com/questions/25872633/using-factor-levels-with-geom-rect

Unfortunately, I didn't get an answer.

Cheers,

Loris

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