I'm not sure exactly what the parents means, but In my opinion there should be a color bar that shows what the lightest color and darkest color on the board mea

I'm not sure exactly what the parents means, but In my opinion there should be a color bar that shows what the lightest color and darkest color on the board means, actually probably two color bars, one for each ring. This would be useful even if color is being used only as a relative measure not as an absolute "number of times loved". Using color as an axis can be notoriously difficult to do properly/clearly (consider linear vs logarithmic mappings or using hue, saturation and/or value together to show one dimensional data) without even getting into aesthetic considerations. Lest you think that I am being too critical, the fact that the graphic designer only modulated color value and not hue and saturation makes this chart better than some that I have seen.