Provides classes for data structures representing plots.
MultiPlotState.java: This class extends PlotState, stores plotting information
for MultiPlot.

Plot3DState.java: This class stores plotting information for 3D plots. It does
not extend PlotState as the structure is considerably different from 2D plot
states.

PlotState: This class is a base class and hold necessary interface and
information for 2D plots, namely MultiPlotState and SinglePlotState. 

SinglePlotState: This class extends PlotState, sotres plotting information for
plots between two variables.