The infographic you posted is misleading, at best. Really, no one eats lettuce for its protein content.
The other thing that jumps out is that pulses (legumes, lentils, beans, peas) are what plant based eaters typically eat when they want protein - and those foods' carbon footprint is tiny: http://www.greeneatz.com/foods-carbon-footprint.html
Add lentils, beans and peas into your chart and remove lettuce if you have a serious interest in representing the food carbon footprint per gram of protein of protein-rich foods.