Or you can forget all the charts and plant by nature's signs.
Here's a nice article on phenological signs
It says Tomatoes can be set out when lily-of-the-valley is in full bloom.
See also applestar's thread on phenological signs, here:
Plant tomatoes, early corn and peppers when dogwoods are in peak bloom or when daylilies start to bloom.
I'm not sure about the lily of the valley, I don't have any, but I know the dogwoods and daylilies haven't happened here yet. My tomatoes have been in the ground a month. Most people always tell me I plant tomatoes too early, but it works for me. They survive and set down roots even though they aren't growing a whole lot, but then when the warm up comes, they take off like crazy, while other people's are still trying to get hardened off and established. I'm the one of the few people in my area to have June tomatoes, especially without a greenhouse or hoophouse or anything...
So you will find tons of different opinions about when to plant tomatoes!