Any major format will work (jpg, png, svg) for me. I have read the related questions on this Wiki but no “export to graphic” option appears for me whenever I click the graph in calc. I am using LibreOffice 4.2.
The new ‘Export as Image’ facility of the context menue of a Chart seemingly does not yet work with everyone of the offered formats. Testing it it worked at least (on my system with 4.3.1.2) for jpg. The “old” procedure of ‘Copy’ and then ‘Paste Special’ (into a Draw document e.g.) will possibly offer an alternative. Of course, I am speaking of a complete chart, not of a single graph (series) contained in it.