This is the reply i got from cie
Dear Sir
Thank you for your email.
Cumulative frequency diagrams, if given in the question, are drawn as curves.
If candidates are asked to draw the cumulative frequency diagram, we will accept either the curve or the straight lines between points and our tolerance with answers accommodates both.
Please contact us if you require further assistance.
Yours sincerely
James Furness
Customer Service Advisor
University of Cambridge International Examinations
E-mail: international@cie.org.uk
Phone: +44 (0)1223 553554
Fax : +44 (0)1223 553558
www:
http://www.cie.org.uk/