@mjlsnombrado, Treemap Custom Visualization is built by Splunk so if there is an issue or enhancement you can also check with Splunk. If you want to make the changes yourself, you will have to edit the following file (this is the compiled version of JS file for the visualization).
Original Source Code is placed under the following path (however, if you make changes to this file, you will have to have all dependent libraries and recompile the complete code. So you should use this code only to understand as to which section does what and where the changed need to be made): $SPLUNK_HOME/Splunk/etc/apps/treemap_app/appserver/static/visualizations/treemap/treemap.js
Even if you make the changes to visualization.js and get it to work, you will need keep the list of changes so that in case there is an upgrade, you would need to sync the code changes manually.