png( "commits_author_day.png", width = 1200, height= 1200 ) 
print( xyplot( commits ~ date | author, data = day_author_data, ylab = "commits / day",
    panel = function(x, y, ...){
        panel.yearlines() 
        panel.xyplot( x, y, pch = "+", ... )
        panel.loess.fill( x, y, lwd = 2, col = "black", ... )
    }, ylim = c(0, 10), subset = ! author %in% c( "r", "mike", "(no author)" ) ) )
dev.off()