Rules of art, part one.
The First rule of art: There are no rules. The second rule of art: Never split the focus and for sure don't stick the focal point in the corner.
Clearly Degas had never taken a workshop. He disobeyed rule #2 and opted for rule #1. In fact, he broke more than a few of the commandments with this piece. I'm a little torn about where to look, girl in orange or girls in the middle or girls in the far corner. It's like a day at the beach. There is a solid unbroken line that goes right of the edge and into the abyss. And... so big fuckin deal, it still works, at least for me. I don't have a problem with choosing where to look, orangey trumps the two girls in the center, in part because of the hard delineation of the red and turquoise wall.
First of all, the idea of the focal point is often abused. Students ask when I'm doing a demo where my focal point is and I usually say, when it tells me I'll let you know. I don't think about the one center of interest, I think about creating a hierarchy of importance, usually a careful balance between 2 or three things. I may not decide which one should be more important until I can get a sense of where my eye is going through a painting. Probably why I like the Degas. And sometimes, there is no focal point at all because it's not the point of the painting.
I teach enough workshops that I often hear, in response to my query, "So, what are you doing there?", "Well, I was told that you never, or you always or you shouldn't...". Lots of rules. A lot of good ones for sure but, as I have said before, show me an art standard and I'll show you someone who broke it with true mettle. I have encountered enough misapplied rules that I just say, don't paint by rules, paint by sight. Let your eyes tell you what to do. Learn the rules and then make your own. Don't let some weekend instructor tell you what to do... except me because I'll tell you the 4 or 5 things you really need to know and then help you find your way.