The link provided by Richard describes the problem well; but, ultimately, it looks like you need to manually set the upper bound of your range axis to account for the upper curve of the spline produced by XYSplineRenderer. For your example above, this might be:
xyplot.getRangeAxis().setUpperBound(22.5);
For practical purposes, you would probably want to calculate the maximum Y value, and either add a percentage to it, or more complicated, calculate a ceiling based on its surrounding points. I would start by adding 10% and see how that goes:
// Iterate data values; use Math.max() to determine maxYValue; then:
xyplot.getRangeAxis().setUpperBound( maxYValue + maxYValue * 0.1);
It's at patch, but it should provide the result you want, depending on the nature of your data and the curves produced connecting data points.