That's really dumb! It's like graphing how much people like bananas and melons using the same axis. If you don't see what's wrong with this, imagine that you don't like melons or bananas. Or that you hate melons and like bananas, but don't like bananas as much as everyone else who's all the way over on the not-melon side. There's no way to express those ideas on this scale, because the only way to like melons less is to like bananas more. This is stupid because how much you like bananas is not related to how much you like melons.
To bring it back to sexuality, being less attracted to one gender does not mean that you have to be more attracted to the other gender. Those variables are independent. What the heck are they doing on the same axis? Really the graph should look like more like this:
What a nice-looking graph. ...Except then this happens: