Let's look at what the area under the line represents:

Rectangle: u × t = ut

Triangle: (v-u) × t ÷ 2

Total: ut + ½vt - ½ut = ½vt + ½ut = ½ (v+u) × t

So the total area is given by s = ½ (v+u) × t (where s is the letter used for displacement).

½ (v+u) is, of course, just the average velocity. In other words, displacement = average velocity × time! This is nothing new...