We have configured iReport to generate the following graph:

The real data points are in blue, the trend line is green. The problems include:

• Too many data points for the trend line
• Trend line does not follow a Bezier curve (spline)

The source of the problem is with the incrementer class. The incrementer is provided with the data points iteratively. There does not appear to be a way to get the set of data. The code that calculates the trend line looks as follows:

import java.math.BigDecimal;
import net.sf.jasperreports.engine.fill.*;

/**
 * Used by an iReport variable to increment its average.
 */
public class MovingAverageIncrementer
  implements JRIncrementer {
  private BigDecimal average;

  private int incr = 0;

  /**
   * Instantiated by the MovingAverageIncrementerFactory class.
   */
  public MovingAverageIncrementer() {
  }

  /**
   * Returns the newly incremented value, which is calculated by averaging
   * the previous value from the previous call to this method.
   * 
   * @param jrFillVariable Unused.
   * @param object New data point to average.
   * @param abstractValueProvider Unused.
   * @return The newly incremented value.
   */
  public Object increment( JRFillVariable jrFillVariable, Object object, 
                           AbstractValueProvider abstractValueProvider ) {
    BigDecimal value = new BigDecimal( ( ( Number )object ).doubleValue() );

    // Average every 10 data points
    //
    if( incr % 10 == 0 ) {
      setAverage( ( value.add( getAverage() ).doubleValue() / 2.0 ) );
    }

    incr++;

    return getAverage();
  }


  /**
   * Changes the value that is the moving average.
   * @param average The new moving average value.
   */
  private void setAverage( BigDecimal average ) {
    this.average = average;
  }

  /**
   * Returns the current moving average average.
   * @return Value used for plotting on a report.
   */
  protected BigDecimal getAverage() {
    if( this.average == null ) {
      this.average = new BigDecimal( 0 );
    }

    return this.average;
  }

  /** Helper method. */    
  private void setAverage( double d ) {
    setAverage( new BigDecimal( d ) );
  }
}

How would you create a smoother and more accurate representation of the trend line?