Enable scripting on the chart:
Then:
def configureChart(data, chart):
from org.jfree.chart.renderer.xy import XYSplineRenderer
plot = chart.getPlot()
renderer = XYSplineRenderer(8)
plot.setRenderer(0, renderer)
That will add shapes to each plot point, but you can remove those:
for i in range(plot.getSeriesCount()):
renderer.setSeriesShapesVisible(i, False)
