Cache the size of the plot area as NSDecimal values to speed up coordinate space conversion. Fixed issue #105.
Merge commit '297abb9c87ebf309514e2211ac56102b353fb3e0'

* commit '297abb9c87ebf309514e2211ac56102b353fb3e0':
  Cache the size of the plot area as NSDecimal values to speed up coordinate space conversion. Fixed issue #105.