I've configured jacoco in the postbuild, I can see the numbers:

however trends chart is empty:

I run jacoco via ant like this:
<target name="test" depends="compile"><jacoco:coverage destfile="${jacoco.result.dir}/jacoco.exec"><testng outputdir="${test.dir}" classpathref="classpath.test" haltonfailure="yes"><xmlfileset dir="${basedir}" includes="testng.xml"/></testng></jacoco:coverage></target>

Any clue ?