P2003 answer (but I'm still curious about what you are calling a "bar chart" -- please let me know).
Step 1 -- calculate the weeks.
1. Insert the column "text1"
2. RightMousebutton on the column header and click on "Customize Fields"
3. Click on Rename and give the column the name "PDQBach"
4. Click on the button labelled "Formula..."
5. Paste the next line into the formula field:
ProjDateDiff(Start,Finish)/2400 & "wks"
2400 is the number of minutes in an hour (60) times the number of hours in a work day (8) times the number of days in a work week (5). Modify as appropriate to your case.
6. Click on OK to exit from the Formula entry.
7. Click on OK to exit from the Customize Fields.
Step 2 -- display it in the Gantt chart
1. Click on Format | Bar styles ...
2. Determine the bar type you want to modify (Task is probably the one you want). Click on it.
3. Click on the tab "Text" below.
4. Choose (left, right, top, bottom, inside) where you want the information to appear.
5. Click on the field and you'll get a drop down. Look for "PDQBach" and click on it.
6. Click on OK.