R: using lattice graphs in functions

I like the lattice graphics package; it produces nice looking and easy to generate graphs. However, I was bitten the other day when a function I'd written to generate a load of density plots wasn't creating the output files I was expecting. They just weren't there at all. After a little bit of unsuccessful poking around, I gave up and generated them manually (it was a stressful day with tight deadlines).

I went back to this issue today and it turns out that the lattice functions 'do not create plots'. You have to print the object created from the lattice function in order to see/output the graph.

This happens automatically on the command line, which is why I was so confused since the code would work fine outside of a function. If you want your graph to be outputted within a function you have to explicitly print it.

e.g.:

function write_density_plot(data, name)
{
    filename = paste(name, '_density_distribution.png', sep="")
    png(filename)
    print(densityplot(data, main=paste(name, 'score distribution'))
    dev.off()
}

filedata = read.delim('myfile', header=T)
write_density_plot(filedata$blorg, 'blorg')
write_density_plot(filedata$blarg, 'blarg')
write_density_plot(filedata$wib, 'wib')