ubuntu - Graphviz Subscript -

i trying graphviz , working , desperately need subscripts in node labels. unfortunately, looking through endless posts people on similar problems seems fit proposed solutions , yet still not working. heres have code:

 digraph g{ execute [label=<ex<sub>2</sub>>]; main -> parse -> execute; main -> init; main -> cleanup; init -> make_string; main -> printf; } 

also run this:

$ dot -tps:cairo -v test.gv -o out.ps 

and output:

> dot - graphviz version 2.26.3 (20100126.1600) activated plugin library: libgvplugin_pango.so.6 using textlayout: textlayout:cairo activated plugin library: libgvplugin_dot_layout.so.6 using layout: dot:dot_layout using render: cairo:cairo using device: ps:cairo:cairo plugin configuration file:     /usr/lib/graphviz/config6         loaded.     render  :  cairo dot fig gd map ps svg tk vml vrml xdot     layout  :  circo dot fdp neato nop nop1 nop2 osage patchwork sfdp twopi     textlayout  :  textlayout     device  :  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     loadimage   :  (lib) eps gd gd2 gif jpe jpeg jpg png ps svg error: unknown html element <sub> on line 1  fontname: "times-roman" resolved to: (ps:pango  times roman,) (pangocairofcfont) "dejavu sans 14"  in label of node execute network simplex:  7 nodes 6 edges maxiter=2147483647 balance=1 network simplex: 7 nodes 6 edges 0 iter 0.00 sec mincross: pass 0 iter 0 trying 0 cur_cross 0 best_cross 0 mincross g: 0 crossings, 0.00 secs. network simplex:  13 nodes 16 edges maxiter=2147483647 balance=2 network simplex: 13 nodes 16 edges 1 iter 0.00 sec routesplines: 6 edges, 18 boxes 0.00 sec using render: cairo:cairo using device: ps:cairo:cairo 

i m not quite familiar cairo or svg renders , quite unsure if packaged graphviz or if separate library together. have ubuntu 12, installed graphviz via apt-get. @ appreciated.

thanks

here's why - the graphviz documentation:

the font markups bold, italic, underlining, subscript , superscript (<b>, <i>, <u>, <sub>; , <sup>) available in versions after 14 october 2011, , available via cairo , svg renderers

your version 2.26.3 january 2010.

i recommend upgrade.

see this answer similar question