[R] How to get R to ignore certain values when analyzing a column in a data table ?

[Daryl Manning]

Apologies if this is in (one of the many) manuals somewhere... Trying  
to switch to R from other stats programs.

Basically, I have a large data table I've dumped from a DB, some of  
the values which are nulls '-' which I've converted to zeros. I've  
read it in using read.table fine.

I want R to ignore the zero values when graphing or doing various  
other calculations.

Is there a way to do this ?

I did try to use NA but kept getting errors that x must be numeric.

thanks in advance,
Daryl.