strange behavior of plotmath

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

It's unexpected but not inexplicable.
The difficulty is that the superscript begins with a unary minus operator. It _must_ have something (number or variable) after it to negate. Since the thing following the '-' is not a number, plotmath has just taken the entire following string as part of an unlikely (but technically possible) variable name "~(mg/L)". 

Rui Barradas' corrected form creates the '-' as a string, preventing the inappropriate mathematical interpretation.

You don't actually _need_ the enclosiong {}, though;

expression(NO[3]^'-'~(mg/L))

works as you intended

S Ellison


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