In Javafx 8, the LineChart does not plot data in the order it was received, it does some sort of very odd 'x-axis sorting' where the lines that connect points are purely determined by their closest counterpart on the x axis. This is quite upsetting when strict path-following is needed (and therefore requires overlapping of lines). I'm attaching two images, one of the java 7 Line Chart, and one of the Java 8 Line chart, along with the source code (easily executable source code). Both examples were created with the same source, simply a different jdk.

My companies application relies heavily on this functionality, and we have been forced to upgrade to java 8 for java 7 bug reasons.

This was in Java 8u20 build 23 specifically