Custom reports using Job Duration for the Y1 axis are reporting data for all schedule types, no matter what is selected in the filter section. For instance, if you want to display the average duration for incremental backups of a particular client or policy and select the correct filters for such, the report returned shows both full and incremental backups.  If you want to identify the maximum duration for the full backups of a client, the results return both fulls & incrementals.  The filter option for both fulls and incrementals is listed, suggesting that you can report on that data, but you cannot.

This was reported to Symantec in ticket #  03396537.  Per the tech assigned to the case, he found a section of notes of the Job Duration field that indicate this is to be expected, so this is not a bug.  The report is "working as designed".  I firmly believe this is a bad design choice on the part of the developers and should be rectified by honoring the users filter selections when reporting Job Duration.