Assuming your plot is saved as p
p
p + opts( legend.position = c(0.9, 0.6), # c(0,0) bottom left, c(1,1) top-right. legend.background = theme_rect(fill = "white", colour = NA))
If you want the legend background partially transparent, change the fill to, e.g., "#ffffffaa".
fill
"#ffffffaa"
Or, building on @Richie Cotton's answer, since opts is deprecated in ggplot2 now (still assuming your plot is defined as p)
p + theme(legend.position = c(0.9, 0.6) ,legend.background = element_rect(fill = "white", colour = NA))