That chart gives the 50% risk of last frost. If you search around, you can get the 90% risk date, which is a couple of weeks later for us in SE Texas. It changes from Feb 27 to Mar 17, which is more accurate, but still leave a slight chance of frost. Try your county agent for a date or NOAA, which I would trust more than any other internet site.
OH my! I guess it isn’t the helpful tool I thought it might be at first! I have a friend whose dad was a longtime amazing gardener, and she told me the date he always used as the “Plant Day.” I think I’ll be ok to use that suggestion also as a rule of thumb!