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Getting certain special symbols into R plots, combined with values that are currently stored in variables, has been an ongoing headache of mine. In particular, plotmath symbols such as the plusminus sign (±), for which the plotmath command is %+%
, had always caused problems due to my inability to parse the R help documentation. For an example, I was trying to get the following simple value (a mean ± 1 SE for a temperature difference) inserted into a plot (a minimalist example):
It turns out that the plotmath operators that are surrounded by percent signs need to be surrounded by empty quote marks to get them to appear correctly inside a bquote()
call. To make the plot above, I used the following commands:
# Define some variables mymean = 1.2345678 mySE = 0.55555 # Make the empty plot plot(x = 0, y = 0, type = 'n') # Assemble a label, using bquote() mylabel = bquote(Delta*italic(T)[max]~.(format(mymean,digits=3))*''%+%''* .(format(mySE,digits=2))*degree*C) # Add mylabel to the plot at x=0, y=0 text(x = 0, y = 0, labels = mylabel, font = 2, cex = 2)
The bquote()
is where the action happens. Inside bquote()
, I have the following arguments:

Delta
gets interpreted as a keyword, and turned in to the greek symbol.  The asterisk makes sure Delta is directly next to whatever follows the asterisk (instead of the ~ tilde symbol, which inserts a blank space).

italic(T)
makes the capital letter T appear in italics. 
[max]
makes whatever is inside the brackets into a subscript. A superscript would be set using the ^ symbol.  The tilde puts a space after the subscript

.(format(mymean,dig=3))
evaluates the variable inside.()
and returns the value. In this case, I have inserted a call toformat()
to make sure the value ofmymean
is displayed with a sensible number of significant digits.  The asterisk leads into the
''%+%''
chunk. The empty quotes around%+%
allow it to work here and be converted into the ± symbol.  The next asterisk leads into
.(format(mySE,digits=2))
, which is another call to.()
used to retrieve and print the numeric value stored inmySE
, with the appropriate number of digits.  The next asterisk leads into
degree
, which is another keyword, and R will convert it into the degree symbol °.  Finally, the next asterisk leads to
C
, which just gets interpreted as the letter C, since it is not a keyword. I could have written a whole word here if desired (Celsius), or even multiple words separated by ~ tildes.
Once mylabel
is assembled, I stick it in the plot at a location defined in the text()
function.
The complete list of keywords and operator symbols available for use in this way can be found by typing ?plotmath
at the R console and perusing the help file.
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