I think ... For each iteration there are 20 individuals. Each gets a score according to how far they get. The horizontal line is the generation. The black l

I think ... For each iteration there are 20 individuals. Each gets a score according to how far they get. The horizontal line is the generation. The black line shows the best in each generation, and green line shows the average over the 20 individuals in that generation. The black line is always above the green, but sometimes crashes way down. The green line doesn't change dramatically, and generally climbs.