You can also add range markers. This example is from the configureChart
extension method.
def configureChart(self, chart):
from org.jfree.chart.plot import ValueMarker
from java.awt import Color, BasicStroke
plot = chart.getPlot()
stroke = BasicStroke(1.0)
# Draws a horizontal red line through 12.4
plot.addRangeMarker(ValueMarker(12.4, Color.RED, stroke))
# Draws a horizontal blue line through 12.225
plot.addRangeMarker(ValueMarker(12.225, Color.BLUE, stroke))
EDIT:
Changed to fit your example with colors and location of markers