[R] gathering denominator under frac

[Troels Ring]

Hi friends - I'm plotting a ratio of bicarbonates i ggplot2 and

ylab(expression(paste(frac("additive BIC","true BIC")))) worked OK - but 
now I have been asked to put the chemistry instead - so I wrote

  ylab(expression(paste(frac("additive",HCO[3]^"-","true",HCO[3]^"-")))) 
- and frac saw that as additive = numerator and HCO3- = denominator and 
the rest was ignored-

So how do I make frac ignore the first ","  and print the fraction as I 
want?


All best wishes
Troels