We I throw the goal "dashboard:report-single" I had an error if I am off line:
BUILD FAILED
File...... C:\maven\home\.maven\cache\maven-dashboard-plugin-1.9-SNAPSHOT\plugin.jelly
Element... j:import
Line...... 210
Column.... -1
file:/C:/maven/home/.maven/cache/maven-dashboard-plugin-1.9-SNAPSHOT/plugin-resources/aggregators/coberturaloc.jelly:32:
-1: <x:parse> Error on line 2 of document : External entity not found: "http://cobertura.sourceforge.net/xml/coverage-0
2.dtd". Nested exception: External entity not found: "http://cobertura.sourceforge.net/xml/coverage-02.dtd".
Total time : 2 seconds
Finished at : Wednesday, November 30, 2005 6:25:22 PM CET

This error is due to the DTD declaration into the file "target\docs\cobertura\coverage.xml":
<!DOCTYPE coverage SYSTEM "http://cobertura.sourceforge.net/xml/coverage-02.dtd">

If your remove this declaration the target dashboard:report-single succed in.
This is because the plugin need to be on line to be able to download the dtd.

I tried to change the aggretors' jelly script but the jelly attribute "validate" doesn't seems to perform:
I replace the part:
<j:when test="${artifactAsFile.exists()}">
<x:parse var="doc" xml="${artifactAsFile}"/>
<x:expr select="count($doc/coverage/packages/package/classes/class/lines/line[@number>0])"/>
</j:when>

with :
<j:when test="${artifactAsFile.exists()}">
<x:parse var="doc" xml="${artifactAsFile}" validate="false"/>
<x:expr select="count($doc/coverage/packages/package/classes/class/lines/line[@number>0])"/>
</j:when>

But I don't resolve my issue