graph : failed to shell execute cmd="dot -Tpng"

Hello

Xhprof with Drupal show list of available runs for detailed reporting
View details work good
but when press graph error appears:
failed to shell execute cmd=" dot -Tpng"

of course graphviz and dot present:
dot -T?
Format: "?" not recognized. Use one of: canon cmap cmapx cmapx_np dot eps fig gd gd2 gif gv imap imap_np ismap jpe jpeg jpg pdf plain plain-ext png ps ps2 svg svgz tk vml vmlz vrml wbmp x11 xdot xlib

Ubuntu 10.10 SMP x86_64 Linux 2.6.35-25-server
php 5.3.3-1ubuntu9.5