Changing the line is definitely the easiest way to do this. I put this on mousePressed to make it easier to test, but it should work from most places:
from java.awt import BasicStroke, Color
chart = event.source.getChart()
plot = chart.getPlot()
plot.setSeparatorStroke(BasicStroke(10))
plot.setSeparatorPaint(Color(255, 255, 255))
plot.setShadowPaint(None)
plot.setOutlineVisible(False)