For PDF, you can specify a text:align:center in the block:
<block style="text-align:center">
<barChart
The above does not work for HTML.
In a forthcoming ZEN Reports you can specify div:
<group name ="Theatres" pagebreak="true" >
<div style="text-align:center">
The above does work for PDF and HTML.
Best Regards,
Jonathan S. Levinson
Senior Software Developer
Object Group
InterSystems
617-621-0600