ggplot2: annotated horizontal lines

Hello,

I'm using geom_hline to add a minimum line to my plot (representing  
the best solution found so far by a search algorithm). I'd like to  
annotate this line with it's numerical value to save trying to read it  
off the graph, but I can't see a clear way to do this - any ideas?

(Alternatively, if this is against the spirit of the grammar of  
graphics, is there a better way to represent the information?)

Cheers,
dave

The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.