Font substitution in SVG output: Helvetica -> Arial ?

[emmanuelito]

In Debian 4, pydot 0.9.6, the follow code:
...
n.set_fontname('Helvetica')
n.set_fontsize(8.0)
...
g.write(path=path, format='svg')

correctly gives XML markup with:
style="font-family:Helvetica;font-size:8.00pt;"


In Debian 5, pydot 0.9.6, the same code render as:
style="font-family:Arial;font-size:8.00;"

(awfull, obviously, and not the same size on screen with Firefox/Mac)

Same python (2.4), same pydot.
Who is responsible for this font substitution ?

Thanks,
Emmanuel