Damn, i thought i was being smart by making my hover signal attached to the QScatterSeries. I guess i'll have to find another way of doing it.
Just a guess, but QScatterSeries might check every point to determine where the mouse is hovering, whereas QLineSeries might just check the bounding rectangle for the entire line.

BTW, QCustomPlot is really the way to go if you want to be able to customize anything about your plots. The way the QChart library is designed, there is absolutely no way to extend it to do something different from what its authors wrote. There are no virtual methods, so you can't derive something new from QScatterSeries, for example, to override the method to change the default behavior. You can't add a new plot type if the standard line, scatter, and other series don't work for your needs. What you see is what you get, and that's all you get.

QCustomPlot is completely customizable. I needed a scatter plot where each dot could have a different size, shape, and color, and where I could supply the data through a QAbstractItemModel without making a copy. I could do all of that with QCustomPlot and none of it with QChart.