Burndown chart report

Hi all,
How can I configure the burndown chart to display work units, say instead of number of activities?
Reason is that we can set work units to activities which report on the relative size of activities. Let's say we have 2 activities: one with 2 work units and one with 8 work units. If I terminates the 2 work units activity, the burndown chart will say 50% of activities are done which is correct. However, only 20% of the "work" is actually done.
I cannot rely only on the consumed time (Everybody knows the 80/20 rule) which provides another complementary insight on the progress. 
Thank you for your help