pdm: repair inventory chart report

this report got broken in 2178cbed (pdm: fix pylint messages and
enable coding style test, 2020-03-09) there's no "Base" portal type.

This report has no test coverage, but the bigger problem is that the
graph from this report is not readable.
7 jobs for erp5-component
Status Job ID Name Coverage
  External
failed ERP5.UnitTest-Master

01:55:45

passed SlapOS.Eggs.UnitTest-Master.Python2

00:43:35

passed SlapOS.Eggs.UnitTest-Master.Python3

00:28:35

failed ERP5.UnitTest-Master

failed ERP5.UnitTest-Master

02:23:39

passed SlapOS.Eggs.UnitTest-Master.Python2

00:36:05

passed SlapOS.Eggs.UnitTest-Master.Python3

01:12:50