[R] strange behavior of plotmath

[Jinsong Zhao]

hi there,

I find the following codes produce strange output.

plot(1:10, xlab = expression(NO[3]^-~(mg/L)))

you will notice that the unit, mg/L is in superscript format. 
That means that "~" is not for space. 
However, the help page of plotmath does not mention this behavior.

Best,
Jinsong
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