  TOPCAT is a widely used desktop application for manipulation of astronomical
catalogues and other tables, which has long provided fast interactive
visualisation features including 1, 2 and 3-d plots, multiple datasets, linked
views, color coding, transparency and more. In Version 4 a new plotting library
has been written from scratch to deliver new and enhanced visualisation
capabilities. This paper describes some of the considerations in the design and
implementation, particularly in regard to providing comprehensible interactive
visualisation for multi-million point datasets.
