implemented an algorithm that does the smarter Y-axis auto ranging. (#1246)

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@7319 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/model/Job.java

@@ -859,6 +859,7 @@ public abstract class Job<JobT extends Job<JobT,RunT>, RunT extends Run<JobT,Run
         domainAxis.setCategoryMargin(0.0);
 
         final NumberAxis rangeAxis = (NumberAxis) plot.getRangeAxis();
+        ChartUtil.adjustChebyshev(dataset,rangeAxis);
         rangeAxis.setStandardTickUnits(NumberAxis.createIntegerTickUnits());
 
         StackedAreaRenderer ar = new StackedAreaRenderer2() {

core/src/main/java/hudson/util/ChartUtil.java

@@ -4,6 +4,8 @@ import hudson.model.AbstractBuild;
 import org.jfree.chart.ChartRenderingInfo;
 import org.jfree.chart.ChartUtilities;
 import org.jfree.chart.JFreeChart;
+import org.jfree.chart.axis.NumberAxis;
+import org.jfree.data.category.CategoryDataset;
 import org.kohsuke.stapler.StaplerRequest;
 import org.kohsuke.stapler.StaplerResponse;
 
@@ -97,6 +99,85 @@ public class ChartUtil {
         rsp.getWriter().println(ChartUtilities.getImageMap( "map", info ));
     }
 
+    /**
+     * Adjusts the Y-axis so that abnormally large value won't spoil the whole chart
+     * by making everything look virtually 0.
+     *
+     * <p>
+     * The algorithm is based on <a href="http://en.wikipedia.org/wiki/Chebyshev%27s_inequality">Chebyshev's inequality</a>,
+     * which states that given any number sequence, nore more than 1/(N^2) values are more than N x stddev away
+     * from the average.
+     *
+     * <p>
+     * So the algorithm is to set Y-axis range so that we can see all data points that are within N x stddev
+     * of the average. Most of the time, Cebyshev's inequality is very conservative, so it shouldn't do
+     * much harm.
+     *
+     * <p>
+     * When the algorithm does kick in, however, we can kick out at most 1 in N^2 data points.
+     * (So for example if N=3 then we can "fix" the graph as long as we only have less than 1/(3*3)=11.111...% bad data.
+     *
+     * <p>
+     * Also see issue #1246.
+     */
+    public static void adjustChebyshev(CategoryDataset dataset, NumberAxis yAxis) {
+        // first compute E(X) and Var(X)
+        double sum=0,sum2=0;
+
+        final int nColumns = dataset.getColumnCount();
+        final int nRows    = dataset.getRowCount();
+        for (int i=0; i<nRows; i++ ) {
+            Comparable rowKey = dataset.getRowKey(i);
+            for( int j=0; j<nColumns; j++) {
+                Comparable columnKey = dataset.getColumnKey(j);
+
+                double n = dataset.getValue(rowKey,columnKey).doubleValue();
+                sum += n;
+                sum2 +=n*n;
+            }
+        }
+
+        double average = sum/(nColumns*nRows);
+        double stddev = Math.sqrt(sum2/(nColumns*nRows)-average*average);
+
+        double rangeMin = average-stddev*CHEBYSHEV_N;
+        double rangeMax = average+stddev*CHEBYSHEV_N;
+
+        // now see if there are any data points that fall outside (rangeMin,rangeMax)
+        boolean found = false;
+        double min=0,max=0;
+        for (int i=0; i<nRows; i++ ) {
+            Comparable rowKey = dataset.getRowKey(i);
+            for( int j=0; j<nColumns; j++) {
+                Comparable columnKey = dataset.getColumnKey(j);
+
+                double n = dataset.getValue(rowKey,columnKey).doubleValue();
+                if(n<rangeMin || rangeMax<n) {
+                    found = true;
+                    continue;   // ignore this value
+                }
+
+                min = Math.min(min,n);
+                max = Math.max(max,n);
+            }
+        }
+
+        if(!found)
+            return; // no adjustment was necessary
+
+        // some values fell outside the range, so adjust the Y-axis
+
+        // if we are ever to extend this method to handle negative value ranges correctly,
+        // the code after this needs modifications
+
+        min = Math.min(0,min);  // always include 0 in the graph
+        max += yAxis.getUpperMargin()*(max-min);
+
+        yAxis.setRange(min,max);
+    }
+
+    public static double CHEBYSHEV_N = 3;
+
     static {
         try {
             new Font("SansSerif",Font.BOLD,18).toString();