The Time History plot option is intended to allow the user to load and show lines in the Time History file format. The time history plot looks different from the SASSI-PREP time history plot because backing Axis for the Time History plot is now the same axis as the Spectrum plot.
Selecting this menu option is the same as the THPLOT command. Many of the 2D Plotting Commands will be able to modify the plot from the command line or allow the user to do batch image manipulation using a Prep File