This chart shows how much a domain’s overall keyword rankings rose or fell compared to the month before. Each of its organic rankings gets a value. We start with position 1 rankings getting a value of 50, position 2 a value of 49, position 15 a value of 35, and so on. We add all of the domain’s ranking values together to get the total shown for every month in the chart. If its overall rankings improved (closer to #1) the chart line would move up and to the right.
We compare those sums from the past month (and 3 months ago) to come up with the “sum of rank changes” that you see in the stat summary above the chart.