Is pgpfplots actually supported by tex4ht? pgfplots was first released in 2008. Eitan died in June 2009.
Certainly nothing has been done to update the tex4ht support for pgfplots since then :(. I hope it will happen at some point, but I suspect it will take a rather intensive research effort to even begin to figure out what needs updating. I hope CVR will prove me wrong :). k
