There are better ways of doing this, but because I don't understand javascript or Timeplot and had to finish fast I did it like this.
I overload the loadXML function of DefaultEventSource to take inn a SPARQL result XML document with the url to an xsl_url. It converts the sparql result with xslt to the format that Timeplot reads.
I didn't need to read XML as well.
You can use the following xslt to transform sparql results.. or write your own.
Code released under GPLv3