Something similar to your issue is that I believe a NPE is thrown when there is a null yField value. So for instance, in the stacked bar charts Movies example, if you change to code to for getMovie to:

Code:
public List<Movies> getMovieData(int firstYear, int size, double min, double scale) {
            List<Movies> movies = new ArrayList<Movies>();
            for (int i = 0; i < size; i++) {
                if (i %3 == 0) {
                    movies.add(new Movies(firstYear + i, Math.floor(Math.max(Math.random() * scale, min)), Math.floor(Math.max(Math.random() * scale, min)),
                            Math.floor(Math.max(Math.random() * scale, min)), Math.floor(Math.max(Math.random() * scale, min))));
                }
                else {

                    movies.add(new Movies(firstYear + i, Math.floor(Math.max(Math.random() * scale, min)), Math.floor(Math.max(Math.random() * scale, min)),
                            Math.floor(Math.max(Math.random() * scale, min)), null));
                }                  
            }
            return movies;
          }
Where the values have been changed from double -> Double. I don't think your workaround would fix it in this case (and I haven't found one myself - I don't know if I'm doing it wrong, but the charts don't seem to work too well with null field values and ignoring them).