r - Using ggplot2, can I insert a break in the axis?

Question

I want to make a bar plot where one of the values is much bigger than all other values. Is there a way of having a discontinuous y-axis? My data is as follows:

df <- data.frame(a = c(1,2,3,500), b = c('a1', 'a2','a3', 'a4'))

p <- ggplot(data = df, aes(x = b, y = a)) + geom_bar() 
p <- p + opts(axis.text.x=theme_text(angle= 90, hjust=1))  + coord_flip()
p

Is there a way that I can make my axis run from 1- 10, then 490 - 500? I can't think of any other way of plotting the data (aside from transforming it, which I don't want to do)

[Edit 2019-05-06]:

8 years later, above code needs to be amended to work with version 3.1.1 of ggplot2 in order to create the same chart:

library(ggplot2)
ggplot(df) + 
  aes(x = b, y = a) +
  geom_col() +
  coord_flip()

Answer 1

As noted elsewhere, this isn't something that ggplot2 will handle well, since broken axes are generally considered questionable.

Other strategies are often considered better solutions to this problem. Brian mentioned a few (faceting, two plots focusing on different sets of values). One other option that people too often overlook, particularly for barcharts, is to make a table:

Looking at the actual values, the 500 doesn't obscure the differences in the other values! For some reason tables don't get enough respect as data a visualization technique. You might object that your data has many, many categories which becomes unwieldy in a table. If so, it's likely that your bar chart will have too many bars to be sensible as well.

And I'm not arguing for tables all the time. But they are definitely something to consider if you are making barcharts with relatively few bars. And if you're making barcharts with tons of bars, you might need to rethink that anyway.

Finally, there is also the axis.break function in the plotrix package which implements broken axes. However, from what I gather you'll have to specify the axis labels and positions yourself, by hand.