double total = 0.0;

                int seriesCount = dataset.getColumnCount();
                for (int series = 0; series < seriesCount; series++) {
                    Number number = dataset.getValue(series, item);
                    if (number != null) {
                        double value = number.doubleValue();
                        if (value < 0.0) {
                            total = total + value;
                            // '+', remember value is negative
                        }
                    }
                }
                minimum = Math.min(minimum, total);