To view a TreeMap report in Google Analytics, navigate to the Acquisition menu, select All Traffic, and then select TreeMap. A treemap report displaying referral traffic by sessions appears.

My best guess to what was causing this is that your TreeMap contained non-static inner class instances and that your outer class wasn't serializable or it contained a member that wasn't.

java -jar treemap.jar. Open the TM3 file that you just generated. Then, on the right-hand side, choose Hierarchy. Click on DEFAULT HIERARCHY and then REMOVE. Then click on SIP and Add. Do the same ...