The only PolarItemRenderer that currently comes with JFreeChart 1.0.13 is DefaultPolarItemRenderer. You can configure it to connect the items and draw a polygon. If some items within a series have null values for X or Y, the renderer produces strange/undefined behavior, i.e. the corresponding point is located at the top-left corner of the plot (see attached screenshot, the blue series).
The problem is, what would the correct behavior be? Linecharts usually skip null-items, leading to a gap in a line. But that's problematic in combination with the fillSeries-feature of this renderer. How can we fill an open polygon? Should filling be disabled automatically if null-items are contained in a series?
The problem is located in DefaultPolarItemRenderer.drawSeries() while building the polygon. Around line 290.