JFreeChart custom x-axis labels

Question

I have multiple series of data.

• Series are years: 2013,2014,2015 etc.
• Data is date+value within a given year.
• Because data needs to be categorized with years, I'm using "day of the year" values on the x axis, range between 1 and 366. So values for a given year look like this: (1,80), (30,100), (60,71) ..... (255,130)

Example diagram:

My problem is that the X axis contains "day of the year" values, but I have to put month names there. Unfortunately, using a simple DateAxis is not an option, because X values are day numbers (not dates), and AFAIK there is no date format that could convert "335" to "December". Another problem with DateAxis is that it represents points in time, and it provides a separate X axis label for any point in time. But I need to write out labels for exact points in time. Namely: only at the begining of the months. What I really want is something like this:

E.g. instead of putting "ticks" at the beginning/end of a month, I want to display regions. What makes it harder is that the months have different lengths. Since February has 29 days in leap years, I think I will have to use fixed points (day of the year values).

Do I have to write a custom axis renderer for this? How? Is there an easier solution for my problem?

Answer 1

import java.awt.Graphics2D;
import java.awt.font.LineMetrics;
import java.awt.geom.Rectangle2D;
import java.util.List;
import java.awt.Color;

import org.jfree.chart.axis.AxisState;
import org.jfree.chart.axis.NumberAxis;
import org.jfree.chart.axis.ValueTick;
import org.jfree.text.TextUtilities;
import org.jfree.ui.RectangleEdge;
import org.jfree.ui.TextAnchor;



public class DayOfYearAxis extends NumberAxis {
    /* Day of the year values for month end days. */
    public static final Integer[] MONTH_LENGTHS = {
            31,29,31,30,31,30,31,31,30,31,30,31
    };
    public static final String[] MONTH_NAMES = {
        "Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"
};

    protected AxisState  drawTickMarksAndLabels(Graphics2D g2,double cursor,Rectangle2D plotArea,Rectangle2D dataArea,RectangleEdge edge) {
        AxisState state = new AxisState(cursor);

        g2.setFont(getTickLabelFont());

        double ol = getTickMarkOutsideLength();
        double il = getTickMarkInsideLength();
        int y = (int)(Math.round(cursor-ol));
        LineMetrics lineMetrics = g2.getFont().getLineMetrics("Ápr", g2.getFontRenderContext());        
        int h = (int) (lineMetrics.getHeight() + 6);

        List<ValueTick> ticks = refreshTicks(g2, state, dataArea, edge);
        state.setTicks(ticks);

        /* Last x point */
        ValueTick tick = ticks.get(ticks.size()-1);     
        float[] prevAnchorPoint = calculateAnchorPoint(tick, cursor,dataArea, edge);
        double xmax = prevAnchorPoint[0];
        double max_day = tick.getValue();

        /* First x point */
        tick = ticks.get(0);
        prevAnchorPoint = calculateAnchorPoint(tick, cursor,dataArea, edge);
        double xmin = Math.round(prevAnchorPoint[0]);
        double min_day = tick.getValue();       
        double days_visible = max_day - min_day + 1;
        /* 0.1 day horizontal gap. */
        double gap = 0.1*(xmax-xmin)/days_visible;

        System.out.println("min_day "+min_day+" max_day"+max_day);

        g2.setFont(getTickLabelFont());
        g2.setColor(Color.BLACK);
        int start_day = 0;
        for (int month=0;month<12;month++) {
            int end_day = start_day + MONTH_LENGTHS[month] - 1;
            System.out.println("start-end "+start_day+" "+end_day);
            if ( (start_day>=min_day) && (start_day<=max_day) && (end_day>=min_day) && (end_day<=max_day) ) {
                double factor_x1 = (start_day - min_day) / days_visible;
                double x1 = xmin + (xmax-xmin)* factor_x1;
                double factor_x2 = (end_day - min_day) / days_visible;
                double x2 = xmin + (xmax-xmin)* factor_x2;
                System.out.println("month="+month+", start_day="+start_day+" end_day="+end_day+" x1="+x1+" x2="+x2);
                g2.setColor(Color.LIGHT_GRAY);
                g2.fill3DRect((int)(x1+gap),y,(int)(x2-x1-2*gap),h,true);
                g2.setColor(Color.BLACK);
                TextUtilities.drawAlignedString(MONTH_NAMES[month], g2, (float)((x1+x2)/2), (float)(y+ol), TextAnchor.TOP_CENTER);
            }           
            start_day += MONTH_LENGTHS[month];
        }
        return state;
    }

}

Usage:

    JFreeChart chart = ChartFactory.createXYLineChart(...);
    DayOfYearAxis doyAxis = new DayOfYearAxis();
    /* optional
    doyAxis.setAutoRange(false);
    doyAxis.setRange(new Range(min_yday, max_yday));
    */
    chart.getXYPlot().setDomainAxis(doyAxis);        

Example output (with hungarian month names):