[NTG-context] How to add margin around standalone MetaPost?

Take the following MetaPost file: beginfig(0) ahlength := 20; linejoin := mitered; drawarrow origin--(40,0) withpen pencircle scaled 5; endfig; Render it into PDF with ConTeXt. Witness the cut out ends of the arrow. How to fix that? Being able to add some margin would be a solution, which may be desirable anyway in some cases. Is that possible? Thanks, Vincent