Alan MacEachren extended these visual variables in 1995 to include display date (time at which change is initiated), frequency (number of times identifiable forms are displayed) and synchronization (correspondence of 2 or more time series) (Slocum et al. 2005).
MacEachren, A. M. and D. R. F. Taylor (eds.) (1994) Visualization in Modern Cartography, Pergamon.
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