Fix label positioning with odd-sized charts.

[Nickardson]

Labels for charts with a size like 199 previously would not be positioned on the top or bottom.

[jsfehler]

@Nickardson I'd assign abs(xa - origin) to a variable so it doesn't need to be calculated twice.

[jsfehler]

@Nickardson Off the top of my head, what happens in the unlikely case a label needs to be positioned at 1, 1? Looks like it'll get placed in the center.

[Nickardson]

@jsfehler Well, the default "fall-through" condition is the label positioned to the right. I don't believe there's a case where the label will be directly on top of the point. Whether that would look good or not, I can't say.