<pre>
> mry <- do.call(rbind, by(movies, round(movies$rating), function(df) {
+     nums <- tapply(df$length, df$year, length)
+     data.frame(rating = round(df$rating[1]), year = as.numeric(names(nums)), 
+         number = as.vector(nums))
+ }))
> p <- ggplot(mry, aes(x = year, y = number, group = rating))
> p + geom_line()
<a href='web/graphics/262760b466c05c5b8fd517622cc7fe63.pdf'><img src='web/graphics/262760b466c05c5b8fd517622cc7fe63.png' /></a>

> p + geom_line(aes(size = rating))
<a href='web/graphics/c006e6c1983f456883ba67ee07eadcdb.pdf'><img src='web/graphics/c006e6c1983f456883ba67ee07eadcdb.png' /></a>

> p + geom_line(aes(colour = rating))
<a href='web/graphics/ec4b647f71f1923d2c549d890b368bf0.pdf'><img src='web/graphics/ec4b647f71f1923d2c549d890b368bf0.png' /></a>

> p + geom_line(aes(colour = rating)) + scale_colour_gradient(low = "red")
<a href='web/graphics/4ee1b927a99c816e359aa4a8db73ce24.pdf'><img src='web/graphics/4ee1b927a99c816e359aa4a8db73ce24.png' /></a>

> p + geom_line(colour = "red", size = 1)
<a href='web/graphics/12d81ebdeabdc858efb7d62a3d6d32bc.pdf'><img src='web/graphics/12d81ebdeabdc858efb7d62a3d6d32bc.png' /></a>

> qplot(year, number, data = mry, group = rating, geom = "line")
<a href='web/graphics/3676f25534ce4db146918a72432671b8.pdf'><img src='web/graphics/3676f25534ce4db146918a72432671b8.png' /></a>

> qplot(year, number, data = mry, group = rating, geom = "path", 
+     statistic = "sortx")
<a href='web/graphics/a6501b0793a36ce5cfd0a38f77155556.pdf'><img src='web/graphics/a6501b0793a36ce5cfd0a38f77155556.png' /></a>

</pre>
