<pre>
> myear <- do.call(rbind, by(movies, movies$year, function(df) data.frame(year = df$year[1], 
+     mean.length = mean(df$length), mean.rating = mean(df$rating))))
> p <- ggplot(myear, aes(x = mean.length, y = mean.rating))
> p + geom_path()
<a href='web/graphics/29d311639e7a64db4e1692d168e180b3.pdf'><img src='web/graphics/29d311639e7a64db4e1692d168e180b3.png' /></a>

> p + geom_path(aes(size = year))
<a href='web/graphics/f8a8991f9a172fb3b69a16e6a6cb208d.pdf'><img src='web/graphics/f8a8991f9a172fb3b69a16e6a6cb208d.png' /></a>

> p + geom_path(aes(colour = year))
<a href='web/graphics/e5ef6fdaa1a93fa9328bf83cc66d79dd.pdf'><img src='web/graphics/e5ef6fdaa1a93fa9328bf83cc66d79dd.png' /></a>

> p + geom_path(aes(size = year)) + scale_size(to = c(1, 3))
<a href='web/graphics/fa3850ee9b223799911f46e3544f3e4a.pdf'><img src='web/graphics/fa3850ee9b223799911f46e3544f3e4a.png' /></a>

> p + geom_path(colour = "green")
<a href='web/graphics/227df22219768dd8e5cf133f2baba797.pdf'><img src='web/graphics/227df22219768dd8e5cf133f2baba797.png' /></a>

> qplot(mean.length, mean.rating, data = myear, geom = "path")
<a href='web/graphics/468a0973c724de11d56c3e46eee02445.pdf'><img src='web/graphics/468a0973c724de11d56c3e46eee02445.png' /></a>

</pre>
