<pre>
> m <- ggplot(movies, aes(y = length, x = rating)) + geom_point() + 
+     scale_y_continuous(limits = c(1, 500))
> m + geom_density_2d()
<a href='web/graphics/ebbf10555d56ba8e3f45c09e55d890a1.pdf'><img src='web/graphics/ebbf10555d56ba8e3f45c09e55d890a1.png' /></a>

> dens <- MASS::kde2d(movies$rating, movies$length)
> densdf <- data.frame(expand.grid(rating = dens$x, length = dens$y), 
+     z = as.vector(dens$z))
> m + geom_contour(aes(z = z), data = densdf)
<a href='web/graphics/6b72442b62c4971923f2759470cfa3e4.pdf'><img src='web/graphics/6b72442b62c4971923f2759470cfa3e4.png' /></a>

> m + geom_density_2d() + scale_y_log10()
<a href='web/graphics/ca069f7d1a533c5a8370b7227cdcccb8.pdf'><img src='web/graphics/ca069f7d1a533c5a8370b7227cdcccb8.png' /></a>

> m + geom_density_2d() + coord_trans(y = "log10")
<a href='web/graphics/ca353c4052d2d6c5e4bd733d822ba15a.pdf'><img src='web/graphics/ca353c4052d2d6c5e4bd733d822ba15a.png' /></a>

> qplot(rating, length, data = movies, geom = c("point", "density2d"), 
+     ylim = c(1, 500))
<a href='web/graphics/03c5324a6560e2e4a727040d8cc21609.pdf'><img src='web/graphics/03c5324a6560e2e4a727040d8cc21609.png' /></a>

</pre>
