I was just going through the list of Chart related issues, and I did not happen to see one for this, so...

I believe the Charts in JavaFX could do with including the ability to add markers onto a chart. These markers could be a single line highlighting a particular value on an axis, or even a range of values(2 lines and a highlighting between them?)

I've found a screenshot of a Swing library, JFreeCharts, showcasing its Markers. As you can see in the attached screenshot, JFreeCharts also had the ability to label these markers.