Thematic Pie Charts with Oracle Views

I am trying to create a thematic Pie Chart from a view within Oracle, but it does not appear to be working for some views.  And example of a view where it does work is below:

CREATE OR REPLACE FORCE VIEW V_DEFECT_DRAFT_RPT_COUNT AS
  SELECT link_id,
    COUNT(*) total_surveys,
    COUNT(report_draft_actual) completed,
    COUNT(*) - COUNT(report_draft_actual) not_completed
  FROM eb_defect_docs
  GROUP BY link_id;

An example where it doesn't work is:

CREATE OR REPLACE VIEW V_DEFECT_LETTER_ACTUAL AS 
SELECT
LINK_ID,
SUM(DECODE(LETTER_ACTUAL,'',1,NULL)) NO_VALUE,
SUM(DECODE(LETTER_ACTUAL,'12-SEP-11 00.00.00',1,NULL)) COLS,
SUM(DECODE(LETTER_ACTUAL,'02-SEP-11 00.00.00',1,NULL)) COLSS,
SUM(DECODE(LETTER_ACTUAL,'08-SEP-11 00.00.00',1,NULL)) COLSSS,
SUM(DECODE(LETTER_ACTUAL,'07-NOV-11 00.00.00',1,NULL)) COLN,
SUM(DECODE(LETTER_ACTUAL,'05-SEP-11 00.00.00',1,NULL)) COLSSSS
FROM EB_DEFECT_DOCS
GROUP BY LINK_ID

The difference appears to be the use of DECODE function (or CASE, same result).  I have also tried variations using COUNT and SUM, but none of these seem to make any difference.  Is there a restriction on the use of SQL in views for pie charts, or is is some other problem?

Regards

Dan