Happens if the data had been deleted while the chart is null. Below is a failing code snippet, more details at SO http://stackoverflow.com/a/24533160/203657

    @Test
    public void testSeriesAddDeleteAdd() {
        Series<String, Number> series = new Series<String, Number>();
        Number value1 = new Integer(5);
        Number value2 = new Integer(6);
        Data<String, Number> point1 = new Data<String, Number>("Something", value1);
        Data<String, Number> point2 = new Data<String, Number>("Something", value2);
        series.getData().add(point1);
        series.getData().add(point2);
        // remove data
        series.getData().clear();

        Number value3 = new Integer(7);
        Data<String, Number> point3 = new Data<String, Number>("Something", value3);
        // add again: here's the NPE
        series.getData().add(point3);
    }