ggplot2 strip text labels facet_wrap
===

I have searched other posts on this topic but can't figure out my problem. I have budgetary data on Brazilian government agencies, and have a classification of agency types. To produce my plot, I do this:  

    g <- ggplot(data=totalex.df, aes(x=year, y=totalex.billions))
    
    g + 
    geom_line(aes(colour=factor(agency.type))) + 
    facet_wrap(agency.type ~  unit, ncol=6) + 
    opts(strip.text.x = theme_text(size=2)) + 
    opts(axis.text.x=theme_text(size=4)) + 
    opts(legend.position="none") + 
    scale_y_sqrt("total expenditure (billions)")

My problem is, my facet labels now contain two pieces of information: the agency type and the name of the unit--[this is a pdf of the graph][1] produced with the above code. I only want it to contain the name of the unit. However, if I drop the agency.type from the facet_wrap command, ggplot loses the order of the agencies. 


  [1]: https://dl.dropbox.com/u/4210336/sqrt_expenditure.pdf