R help - Mar 2017 - ggplot expressions

Dear friends - here is an example of something that I find annoying

library(ggplot2)
DDF <- data.frame(x=x<-seq(1,10),y=x^2)
G <- ggplot(data=DDF,aes(x=x,y=y)) + geom_point()
GG<-G+ylab(expression(paste("Total ",CO[2]," halved")))
GG
GG+annotate("text",x=5,y=50,label=expression(paste("Total
",CO[2],"
halved")))

On my system, R version 3.3.2 (2016-10-31) Windows - the third line 
produces the wanted formatting of CO2  with "2" lowered - while the
last
line produces an error  "invalid 'type' (expression) of
argument" - with
exactly the same string - how comes?

Beautiful ggplot is not exactly low in documentation but finding the 
right stuff is not always easy (for me:-))

Best wishes
Troels Ring, MD
Aalborg, Denmark

> On Mar 20, 2017, at 4:42 AM, Troels Ring <tring at gvdnet.dk> wrote:
> 
> Dear friends - here is an example of something that I find annoying
> 
> library(ggplot2)
> DDF <- data.frame(x=x<-seq(1,10),y=x^2)
> G <- ggplot(data=DDF,aes(x=x,y=y)) + geom_point()
> GG<-G+ylab(expression(paste("Total ",CO[2],"
halved")))
> GG
> GG+annotate("text",x=5,y=50,label=expression(paste("Total
",CO[2]," halved")))
> 
> On my system, R version 3.3.2 (2016-10-31) Windows - the third line
produces the wanted formatting of CO2  with "2" lowered - while the
last line produces an error  "invalid 'type' (expression) of
argument" - with exactly the same string - how comes?
> 
> Beautiful ggplot is not exactly low in documentation but finding the right
stuff is not always easy (for me:-))
As the Posting Guide recommends, the question should include the complete text
of the error message:

"Error in stats::complete.cases(df[, vars, drop = FALSE]) : 
  invalid 'type' (expression) of argument"

In this case, however I find that message completely opaque and the results of
traceback() likewise uninformative, so going back to the help pages, always a
good place to start, notice that the last example on `?annotate` is isomorphic
to you problem, so try using the curiously unlisted `parse`-parameter to
annotate. The parse parameter according to the documentation is being passed to
`layer`, but `?layer` doesn't describe a parse parameter so I think it's
a defect (somewhere) in the ggplot2 help pages.

GG + annotate("text", x=5, y=50, label =
"Total~CO[2]~halved", parse=TRUE)

I have removed the call to `paste` since I find that it's use inhibits
learning to use expressions most economically. I think it's better to use
the tilde rather than assembling a mishmash of quoted and unquoted
sub-expressions.

-- 
David.


> 
> Best wishes
> Troels Ring, MD
> Aalborg, Denmark
> 
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David Winsemius
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