Changing legend position/direction using opts() has no effect

Creating the following plot results in the legend being placed vertically, on the right, rather than on the bottom horizontally as indicated in the call to opts():

dat <- data.frame(x = runif(10), y = runif(10), 
                  grp = rep(letters[1:2],each = 5))

ggplot(data = dat, aes(x = x, y = y, colour = grp)) + 
  geom_point() + 
  opts(legend.position = "bottom", legend.direction = "horizontal") + 
  theme_bw()

How do I get the l开发者_运维技巧egend in the correct spot?

The problem is that that theme_bw() is placed after the call to opts(), and resets some defaults. Just place theme_bw() before opts():

ggplot(data = dat, aes(x = x, y = y, colour = grp)) + 
  geom_point() + 
  theme_bw() +
  opts(legend.position = "bottom", legend.direction = "horizontal")

Note: Since version 0.9.2 opts has been replaced by theme:

theme(legend.position = "bottom", legend.direction = "horizontal")