Question

I’m using jFreeChart 1.0.14. I have graph with a horizontal DateAxis. And I’m trying to center the tick labels so that they would be within two subsequent ticks.

JFreeChart aligns tick labels in common case this way:

----+------------+------------+----
   Mon          Tue          Wed

But I’d like to align tick labels this way:

----+------------+------------+------------+----
         Mon          Tue          Wed

The tick labels on images are aligned by hand in graphic editor by editing the original image. It’s not output of jFreeChart library.

Is there any way, how to do it through the API of DateAxis? Any help is appreciated… 🙂

Thanks a lot for any help or idea.
Honza (sporak)

Answer 1

Since nobody helped me any trick, I tried to overide DateAxis to get Axis with tick labels alignment. I named it ‘AlignedDateAxis‘. It extends DateAxis and overrides only methods that are used for tick labels rendering. It can render tick labels in two ways: in standard way that places labels under ticks, and in required way that centers labels in the middle of interval.

Because I don’t know much about JFreeCharts library understanding some classes and methods wasn’t easy for me. I hope it will work fine in all common cases.
My class contains javadoc and comments only in my native language, so I won’t post them complete here.

package x.y.z;

import java.awt.Graphics2D;
import java.awt.Shape;
import java.awt.geom.Line2D;
import java.awt.geom.Rectangle2D;
import java.util.Date;
import java.util.List;

import org.jfree.chart.axis.AxisState;
import org.jfree.chart.axis.DateAxis;
import org.jfree.chart.axis.DateTick;
import org.jfree.chart.axis.DateTickUnit;
import org.jfree.chart.axis.TickType;
import org.jfree.chart.axis.ValueTick;
import org.jfree.text.TextUtilities;
import org.jfree.ui.RectangleEdge;

/**
 * Extension of DateAxis for jFreeChart graphs that can render tick labels in two ways: 
 *   1) labels under ticks (common way, same as DateAxis)
 *   2) labels in the middle of interval (new feature)
 * @author Honza Spurny, Czech Republic
 * @version 1.0
 */
public class AlignedDateAxis extends DateAxis {

    /**
     * Tick labels alignment setting.
     */
    private TickLabelPosition tickLabelPosition = TickLabelPosition.DEFAULT_VALUE;

    /**
     * Value for interface Serializable.
     */
    static private final long serialVersionUID = 1;

    // ***********************
    // ***      Enums      ***
    // ***********************

    /**
     * Tick label alignment modes.
     */
    public enum TickLabelPosition {
        /**
         * Tick label is rendered under/next by own tick. 
         * (common rendering same as in DateAxis)
         */
        INTERVAL_START,
        /**
         * Tick label is placed in the middle of interval
         * (between two subsequent ticks) 
         */
        INTERVAL_MIDDLE;

        static public final TickLabelPosition DEFAULT_VALUE = INTERVAL_START;
    }

    // ******************************
    // ***      Constructors      ***
    // ******************************

    /**
     * Default constructor.
     */
    public AlignedDateAxis() {
        super();
    }

    /**
     * Constructor.
     * @param tickLabelPos tick label alignment mode setting for this axis.
     */
    public AlignedDateAxis(TickLabelPosition tickLabelPos) {
        this();
        this.setTickLabelPosition(tickLabelPos);
    }

    // *********************************
    // ***      GET/SET methods      ***
    // *********************************

    public TickLabelPosition getTickLabelPosition() {
        return this.tickLabelPosition;
    }

    public void setTickLabelPosition(TickLabelPosition value) {
        this.tickLabelPosition = value;
    }

    // *******************************************************
    // ***      Overrided methods for label rendering      ***
    // *******************************************************

    /**
     * Auxiliary method to calculate tick label position of given tick.
     * @param tick tick we need calculate label position for (DateTick is expected)
     * @param cursor the cursor
     * @param dataArea area to draw the ticks and labels in
     * @param edge edge of dataArea
     * @return Returns coordinates [x,y] where label should be placed.
     */
    @Override
    protected float[] calculateAnchorPoint(ValueTick tick, double cursor, Rectangle2D dataArea, RectangleEdge edge) {
        float[] resultAnchor = super.calculateAnchorPoint(tick, cursor, dataArea, edge);

        // Time of tick
        Date tickDate = (tick instanceof DateTick) ? 
                        ((DateTick) tick).getDate() : 
                        new Date((long)tick.getValue());

        // Tick label shift.
        // (for INTERVAL_START it is 0, for INTERVAL_MIDDLE it is calculated)
        double labelShift;

        switch (this.getTickLabelPosition()) {
            case INTERVAL_MIDDLE:
                // Getting next tick value...
                DateTickUnit unit = this.getTickUnit();
                Date nextTickDate = unit.addToDate(tickDate, this.getTimeZone());
                double nextTickVal = this.valueToJava2D(nextTickDate.getTime(), dataArea, edge);

                // Shifting label in between ticks...
                labelShift = (nextTickVal - resultAnchor[0]) / 2;
                break;

            case INTERVAL_START:
            default:
                labelShift = 0;
                break;
        }

        // Edge defines which coordinate is shifted.
        if (RectangleEdge.isTopOrBottom(edge)) {
            resultAnchor[0] += labelShift;
        } else if (RectangleEdge.isLeftOrRight(edge)) {
            resultAnchor[1] += labelShift;
        }

        return resultAnchor;
    }

    /**
     * Renders this axis with ticks and labels.
     * @param g2 graphics to draw the axis in.
     * @param cursor the cursor
     * @param plotArea area to draw the chart in
     * @param dataArea area to draw this axis in
     * @param edge edge of dataArea
     * @return Returns state of axis.
     */
    @Override
    protected AxisState drawTickMarksAndLabels(Graphics2D g2, double cursor, Rectangle2D plotArea, Rectangle2D dataArea, RectangleEdge edge) {
        AxisState state = new AxisState(cursor);
        if (this.isAxisLineVisible()) this.drawAxisLine(g2, cursor, dataArea, edge);

        List<DateTick> ticks = this.refreshTicks(g2, state, dataArea, edge);
        state.setTicks(ticks);
        g2.setFont(this.getTickLabelFont());

        for (DateTick tick: ticks) {        
            if (this.isTickLabelsVisible()) {
                g2.setPaint(this.getTickLabelPaint());
                float anchorPoint[] = this.calculateAnchorPoint(tick, cursor, dataArea, edge);

                // TextUtilities.drawRotatedString(tick.getText(), g2, anchorPoint[0], anchorPoint[1], tick.getTextAnchor(), tick.getAngle(), tick.getRotationAnchor());
                // Commented code above is original code from DateAxis.

                // ---[Override]---
                // Position of tick label is shifted so it can point outside the dataArea.
                // We have to check whether the tick label on this position is drawable into the given area.                            
                Shape labelBounds = TextUtilities.calculateRotatedStringBounds(tick.getText(), g2, anchorPoint[0], anchorPoint[1], tick.getTextAnchor(), tick.getAngle(), tick.getRotationAnchor());
                double labelEdge = (RectangleEdge.isTopOrBottom(edge)) ? (labelBounds.getBounds2D().getMaxX()) : (labelBounds.getBounds2D().getMaxY());
                double dataAreaBound = (RectangleEdge.isTopOrBottom(edge)) ? (dataArea.getMaxX()) : (dataArea.getMaxY());

                // Magic constant 5: tick label can be rendered although it exceeds area edge for max. 5px
                // (it still looks good - visualy tested :-)
                if ((labelEdge - 5) <= dataAreaBound) {
                    TextUtilities.drawRotatedString(tick.getText(), g2, anchorPoint[0], anchorPoint[1], tick.getTextAnchor(), tick.getAngle(), tick.getRotationAnchor());
                }
                // ---[/Override]---
            }

            if ( (this.isTickMarksVisible() && tick.getTickType().equals(TickType.MAJOR)) || (this.isMinorTickMarksVisible() && tick.getTickType().equals(TickType.MINOR)) ) {
                double ol = tick.getTickType().equals(TickType.MINOR) ? this.getMinorTickMarkOutsideLength() : this.getTickMarkOutsideLength();
                double il = tick.getTickType().equals(TickType.MINOR) ? this.getMinorTickMarkInsideLength() : this.getTickMarkInsideLength();
                float tickVal = (float)this.valueToJava2D(tick.getValue(), dataArea, edge);
                Line2D mark = null;
                g2.setStroke(this.getTickMarkStroke());
                g2.setPaint(this.getTickMarkPaint());
                if(edge == RectangleEdge.LEFT) {
                    mark = new Line2D.Double(cursor - ol, tickVal, cursor + il, tickVal);
                } else if(edge == RectangleEdge.RIGHT) {
                    mark = new Line2D.Double(cursor + ol, tickVal, cursor - il, tickVal);
                } else if(edge == RectangleEdge.TOP) {
                    mark = new Line2D.Double(tickVal, cursor - ol, tickVal, cursor + il);
                } else if(edge == RectangleEdge.BOTTOM) {
                    mark = new Line2D.Double(tickVal, cursor + ol, tickVal, cursor - il);
                }
                g2.draw(mark);
            }
        }

        if (this. isTickLabelsVisible()) {
            double used = 0.0;            
                if (edge == RectangleEdge.LEFT) {
                    used += this.findMaximumTickLabelWidth(ticks, g2, plotArea, this.isVerticalTickLabels());
                    state.cursorLeft(used);
                } else if (edge == RectangleEdge.RIGHT) {
                    used = this.findMaximumTickLabelWidth(ticks, g2, plotArea, this.isVerticalTickLabels());
                    state.cursorRight(used);
                } else if (edge == RectangleEdge.TOP) {
                    used = this.findMaximumTickLabelHeight(ticks, g2, plotArea, this.isVerticalTickLabels());
                    state.cursorUp(used);
                } else if (edge == RectangleEdge.BOTTOM) {
                    used = this.findMaximumTickLabelHeight(ticks, g2, plotArea, this.isVerticalTickLabels());
                    state.cursorDown(used);
                }
        }

        return state;
    }
}

This axis-class (AlignedDateAxis) works fine for me and I’m using it in my project.

Honza (sporak)