/**
     * This method update the chart when any of the selectors change. The data needed (selector values and
     * bar chart object) is stored in 
     */
    protected void updateChart() {
        // the animation routine is unfortunately buggy and will cause the chart
        // to not update correctly, so we disable it
        this.sightingsChart.setAnimated(false);

        // create the main data series for the bar chart which will contain
        // all our sighting counts for each time step
        XYChart.Series<String,Number> series = new XYChart.Series<>();
        // clear all old data series from the chart (if there are any)
        this.sightingsChart.getData().clear();
        // add our main data series to the chart
        this.sightingsChart.getData().add(series); 

        // we give the main series the name "All Sightings"
        series.setName("All Sightings");
        // get the lower and higher bounds and get an iterator as the result of a range query to the skip list
        Date min = this.minSelectedDate == null ? this.dataSkipList.getSmallestKey() : this.minSelectedDate;
        Date max = this.maxSelectedDate == null ? this.dataSkipList.getSmallestKey() : this.maxSelectedDate; 
        Iterator<SquirrelSighting> iter = this.dataSkipList.getRange(min, max);

        // create a hash table that we use to store attribute value counts for the selected attribute
        HashMap<String,Integer> attributeValueCount = new HashMap<String,Integer>();
        // create a hash table to store data series for every value for the selected attribute
        HashMap<String,XYChart.Series<String,Number>> attributeValueSeries = new HashMap<String,XYChart.Series<String,Number>>();
        Date currentDate = null;
        int count = 0;
        while (iter.hasNext()) {
            SquirrelSighting currentSighting = iter.next();
            if (currentDate == null)
                currentDate = currentSighting.getDateTime();
            // create new data points when we jump from one date to another
            if (currentDate != null && !currentDate.equals(currentSighting.getDateTime())) {
                // currentDate = currentSighting.getDateTime();
                // create a new data point for the main series
                series.getData().add(new XYChart.Data<>(dateFormat.format(currentDate), count));
                // and for every series for attribute values that we found in the data
                for (String attributeValue : attributeValueCount.keySet()) {
                    if (!attributeValueSeries.containsKey(attributeValue)) {
                        attributeValueSeries.put(attributeValue, new XYChart.Series<String,Number>());
                        attributeValueSeries.get(attributeValue).setName(this.selectedAttribute + ":" + attributeValue);
                        this.sightingsChart.getData().add(attributeValueSeries.get(attributeValue));
                    }
                    XYChart.Series<String,Number> attributeSeries = attributeValueSeries.get(attributeValue);
                    attributeSeries.getData().add(new XYChart.Data<String,Number>(dateFormat.format(currentDate), attributeValueCount.get(attributeValue)));
                }
                // reset both counts for the main series and all other series
                count = 0;
                for (String attributeValue : attributeValueCount.keySet()) {
                    attributeValueCount.put(attributeValue, 0);
                }
                currentDate = currentSighting.getDateTime();
            }
            // increase count for main data series
            count++;
            //  find the selected attribute
            if (this.selectedAttribute != "none" && this.selectedAttribute != null) {
                // add the value to the hash table if not present, then increase the count
                String attributeValue = currentSighting.getAttributeValue(this.selectedAttribute);
                if (!attributeValueCount.containsKey(attributeValue))
                    attributeValueCount.put(attributeValue, 0);
                attributeValueCount.put(attributeValue, attributeValueCount.get(attributeValue) + 1);
            }
        }
        // and once more for the last series
        series.getData().add(new XYChart.Data<>(dateFormat.format(currentDate), count));
        for (String attributeValue : attributeValueCount.keySet()) {
            if (!attributeValueSeries.containsKey(attributeValue)) {
                attributeValueSeries.put(attributeValue, new XYChart.Series<String,Number>());
                attributeValueSeries.get(attributeValue).setName(this.selectedAttribute + ":" + attributeValue);
                this.sightingsChart.getData().add(attributeValueSeries.get(attributeValue));
            }
            XYChart.Series<String,Number> attributeSeries = attributeValueSeries.get(attributeValue);
            attributeSeries.getData().add(new XYChart.Data<String,Number>(dateFormat.format(currentDate), attributeValueCount.get(attributeValue)));
        }
    }