There is a per-day chart:
- http://pi.uk.com/bitcoin/charts/n-transactions
There is an average transactions per block statistic (previous 1,000 blocks):
- http://blockexplorer.com/q/avgtxnumber
But specifically a transactions per block, I've not seen that charted.
The data is visible from a site:
- http://pi.uk.com/bitcoin/blocks
and also from:
- http://abe.john-edwin-tobey.org/chain/Bitcoin?count=2016
(which comes from ABE which can store the data in MySQL, PostgreSQL and elsewhere):
- https://bitcointalksearch.org/topic/update-abe-agpl-block-explorer-on-github-16141
So to go that further and graph it by the amount of time for the block is something you're probably the first to (publicly) request.