Susan Bell Trickett J. Gregory Trafton Spatial Transformations in Graph Comprehension. 372-375 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2980&spage=372 2004 conf/diagrams/2004 Diagrams db/conf/diagrams/diagrams2004.html#TrickettT04