Scalable Vector Graphics
SVG is an internet standard based on XML for vector graphics, understood by most browsers and many other applications.
ELKI uses SVG for visualization and Batik as screen renderer and for conversion to many formats such as postscript and PDF.
The packages de.lmu.ifi.dbs.elki.visualization.svg and de.lmu.ifi.dbs.elki.visualization.batikutil include additional code useful for SVG and Batik in particular. See their JavaDoc for an overview of the added functionality.
Inkscape is an excellent and free graphics program for editing SVG. This is very useful for preparing graphics for printing!