java.lang.AssertionError: not comparable

[boyang9602]

1. Daikon version: 5.7.2
2. Description:
   I ran daikon on "tests/org/jfree/chart/renderer/category/junit/AbstractCategoryItemRendererTests.java" of Chart project, which is the defects4j, bug 1 version with a plausible patch.
   The exact steps:
   1). Checkout the Chart1b from defects4j: a. clone and compile defects4j(https://github.com/rjust/defects4j), b. defects4j checkout -p Chart -v 1b -w /workdir
   2). Patch the Chart project with the plausible patch below

--- a/source/org/jfree/chart/renderer/category/AbstractCategoryItemRenderer.java
+++ b/source/org/jfree/chart/renderer/category/AbstractCategoryItemRenderer.java 
@@ -1794,9 +1794,6 @@ public abstract class AbstractCategoryItemRenderer extends AbstractRenderer
         }
         int index = this.plot.getIndexOf(this);
         CategoryDataset dataset = this.plot.getDataset(index);
-        if (dataset != null) {
-            return result;
-        }
         int seriesCount = dataset.getRowCount();
         if (plot.getRowRenderingOrder().equals(SortOrder.ASCENDING)) {
             for (int i = 0; i < seriesCount; i++) {

3). Comment all the test cases except "public void test2947660()"
4). Compile Chart: 1. cd /workdir 2. defects4j compile
5). Generate decls-DynComp file: java -cp /workdir/build/:/workdir/build-tests:/workdir/lib/:$DAIKONDIR/daikon.jar daikon.DynComp junit.textui.TestRunner org.jfree.chart.renderer.category.junit.AbstractCategoryItemRendererTests
6). Generate dtrace file: java -cp /workdir/build/:/workdir/build-tests:/workdir/lib/:$DAIKONDIR/daikon.jar daikon.Chicory --comparability-file=TestRunner.decls-DynComp junit.textui.TestRunner org.jfree.chart.renderer.category.junit.AbstractCategoryItemRendererTests
7). Generate invariants: java -cp -Xmx8192M -ea $DAIKONDIR/daikon.jar daikon.Daikon TestRunner.dtrace.gz

All steps above worked well except step 7.
3. Attached is a compiled Chart project, containing the source code, compiled files, decls-Comp file, dtrace file, etc. So you can skip step 1 to 4 and use it directly.
4. Jdk/jre version:
openjdk version "1.8.0_222"
OpenJDK Runtime Environment (build 1.8.0_222-8u222-b10-1ubuntu1~18.04.1-b10)
OpenJDK 64-Bit Server VM (build 25.222-b10, mixed mode)
System version: Ubuntu 18.04.3
5. Other information:
Error message:

Exception in thread "main" java.lang.AssertionError: not comparable
org.jfree.chart.util.GradientPaintTransformType.HORIZONTAL.name org.jfree.chart.util.GradientPaintTransformType.CENTER_VERTICAL.name at ppt org.jfree.chart.util.GradientPaintTransformType:::OBJECT
at daikon.inv.Equality.<init>(Equality.java:125)
at daikon.PptSliceEquality.instantiate_from_pairs(PptSliceEquality.java:236)
at daikon.PptTopLevel.mergeInvs(PptTopLevel.java:3637)
at daikon.PptTopLevel.mergeInvs(PptTopLevel.java:3467)
at daikon.Daikon.createUpperPpts(Daikon.java:1737)
at daikon.Daikon.process_data(Daikon.java:2337)
at daikon.Daikon.mainHelper(Daikon.java:772)
at daikon.Daikon.main(Daikon.java:657)

I debug daikon and found that the data leads to fail is a list contains 3 VarComparabilityImplicit objects with different base values. I also found these values are equal to the comparability value of variables in dtrace file. So I wonder how these values are computed and why they are different. If they should be different, why they are put into the same collection to compare.