* @return The z range.
     */
    public Range getZValueRange(Range x, Range y) {

        double minX = x.getLowerBound();
        double minY = y.getLowerBound();
        double maxX = x.getUpperBound();
        double maxY = y.getUpperBound();

        double zMin = 1.e20;
        double zMax = -1.e20;
        for (int k = 0; k < this.zValues.length; k++) {
            if (this.xValues[k].doubleValue() >= minX
                && this.xValues[k].doubleValue() <= maxX
                && this.yValues[k].doubleValue() >= minY
                && this.yValues[k].doubleValue() <= maxY) {
                if (this.zValues[k] != null) {
                    zMin = Math.min(zMin, this.zValues[k].doubleValue());
                    zMax = Math.max(zMax, this.zValues[k].doubleValue());
                }
            }
        }

        return new Range(zMin, zMax);
    }

    /**
     * Returns the minimum z-value.
     *
     * @param minX  the minimum x value.
     * @param minY  the minimum y value.
     * @param maxX  the maximum x value.
     * @param maxY  the maximum y value.
     *
     * @return The minimum z-value.
     */
    public double getMinZValue(double minX, 
                               double minY, 
                               double maxX, 
                               double maxY) {

        double zMin = 1.e20;
        for (int k = 0; k < this.zValues.length; k++) {
            if (this.zValues[k] != null) {
                zMin = Math.min(zMin, this.zValues[k].doubleValue());
            }
        }
        return zMin;

    }

    /**
     * Returns the number of series.
     * <P>
     * Required by XYDataset interface (this will always return 1)
     *
     * @return 1.
     */
    public int getSeriesCount() {
        return 1;
    }

    /**
     * Returns the name of the specified series.
     *
     * Method provided to satisfy the XYDataset interface implementation
     *
     * @param series must be zero.
     *
     * @return The series name.
     */
    public Comparable getSeriesKey(int series) {
        if (series > 0) {
            throw new IllegalArgumentException("Only one series for contour");
        }
        return this.seriesKey;
    }

    /**
     * Returns the index of the xvalues.
     *
     * @return The x values.
     */
    public int[] getXIndices() {
        return this.xIndex;
    }

    /**
     * Returns the x values.
     *
     * @return The x values.
     */
    public Number[] getXValues() {
        return this.xValues;
    }

    /**
     * Returns the x value for the specified series and index (zero-based 
     * indices).  Required by the {@link XYDataset}.
     *
     * @param series  must be zero;
     * @param item  the item index (zero-based).
     *
     * @return The x value.
     */
    public Number getX(int series, int item) {
        if (series > 0) {
            throw new IllegalArgumentException("Only one series for contour");
        }
        return this.xValues[item];
    }

    /**
     * Returns an x value.
     *
     * @param item  the item index (zero-based).
     *
     * @return The X value.
     */
    public Number getXValue(int item) {
        return this.xValues[item];
    }

    /**
     * Returns a Number array containing all y values.
     *
     * @return The Y values.
     */
    public Number[] getYValues() {
        return this.yValues;
    }

    /**
     * Returns the y value for the specified series and index (zero-based 
     * indices).  Required by the {@link XYDataset}.
     *
     * @param series  the series index (must be zero for this dataset).
     * @param item  the item index (zero-based).
     *
     * @return The Y value.
     */
    public Number getY(int series, int item) {
        if (series > 0) {
            throw new IllegalArgumentException("Only one series for contour");
        }
        return this.yValues[item];
    }

    /**
     * Returns a Number array containing all z values.
     *
     * @return The Z values.
     */
    public Number[] getZValues() {
        return this.zValues;
    }

    /**
     * Returns the z value for the specified series and index (zero-based 
     * indices).  Required by the {@link XYDataset}
     *
     * @param series  the series index (must be zero for this dataset).
     * @param item  the item index (zero-based).
     *
     * @return The Z value.
     */
    public Number getZ(int series, int item) {
        if (series > 0) {
            throw new IllegalArgumentException("Only one series for contour");
        }
        return this.zValues[item];
    }

    /**
     * Returns an int array contain the index into the x values.
     *
     * @return The X values.
     */
    public int[] indexX() {
        int[] index = new int[this.xValues.length];
        for (int k = 0; k < index.length; k++) {
            index[k] = indexX(k);
        }
        return index;
    }

    /**
     * Given index k, returns the column index containing k.
     *
     * @param k index of interest.
     *
     * @return The column index.
     */
    public int indexX(int k) {
        int i = Arrays.binarySearch(this.xIndex, k);
        if (i >= 0) {
            return i;
        } 
        else {
            return -1 * i - 2;
        }
    }


    /**
     * Given index k, return the row index containing k.
     *
     * @param k index of interest.
     *
     * @return The row index.
     */
    public int indexY(int k) { // this may be obsolete (not used anywhere)
        return (k / this.xValues.length);
    }

    /**
     * Given column and row indices, returns the k index.
     *
     * @param i index of along x-axis.
     * @param j index of along y-axis.
     *
     * @return The Z index.
     */
    public int indexZ(int i, int j) {
        return this.xValues.length * j + i;
    }

    /**
     * Returns true if axis are dates.
     * 
     * @param axisNumber The axis where 0-x, 1-y, and 2-z.
     * 
     * @return A boolean.
     */
    public boolean isDateAxis(int axisNumber) {
        if (axisNumber < 0 || axisNumber > 2) {
            return false; // bad axisNumber
        }
        return this.dateAxis[axisNumber];
    }

    /**
     * Sets the names of the series in the data source.
     *
     * @param seriesKeys  the keys of the series in the data source.
     */
    public void setSeriesKeys(Comparable[] seriesKeys) {
        if (seriesKeys.length > 1) {
            throw new IllegalArgumentException(
                "Contours only support one series"
            );
        }
        this.seriesKey = seriesKeys[0];
        fireDatasetChanged();
    }

}