Through lack of recent practice I'd forgotten how to embed a Google Ngram chart in a post, and in my search for where I'd read it on Meta - which I finally found here - I came across this old question. As Google Ngrams now have substantially added functionality, I thought it would be worthwhile to post an updated answer.
You can now easily compare frequencies between the British and American English corpora in the one chart. In the "comma-separated phrases" field, insert the :
corpus selection operator after the search term, followed by the shorthand for the corpus (e.g. eng_us_2012
), add a comma and repeat the search term with section operator and the shorthand for the other corpus (e.g. eng_gb_2012
). You can do this with more than one search term, to see which appears more frequently in each corpus.
As an example, compare the expressions take a walk and go for a walk. A basic Google Ngram using the search {take a walk,go for a walk} shows that the former has always occurred more frequently, but the latter has been growing in usage:

However, if I wanted to look at transatlantic differences in frequency, I'd search for {take a walk:eng_us_2012,go for a walk:eng_us_2012,take a walk:eng_gb_2012,go for a walk:eng_gb_2012}. The resulting chart reveals that take a walk has declined in frequency in the GB corpus over time, to such an extent that go for a walk has been the more common term (albeit narrowly) since about the mid-1930s:

The Google Ngram Viewer info page provides not only the shorthand codes for the different corpora, but also a summary of all the other useful "Advanced Usage" options, including:
- Wildcard search (shows the top 10 substitutions for the word, e.g.
University of
*
)
- Inflection search (e.g. "book_INF a hotel" gives results for "book",
"booked", "books", and "booking")
- Part-of-speech Tags (really useful! E.g. differentiate tackle_VERB vs
tackle_NOUN, or find all expressions of the form "read (determiner)
book" with read _DET_ book)
and a range of even more advanced and complicated search functions!