I loaded an UNSMRY file and a .hist file and tried to make a comparison plot with them. If the UNSMRY file is the active file, then Tecplot RS shows a plot but no comparison, and if the .hst file is the active file then Tecplot RS does show a comparison plot. Why does Tecplot RS not show a comparison plot when the UNSMRY file is active?
This can happen if the two datasets have differing variable lists. When one of the two datasets is active and plotted on the screen Tecplot RS will look at the variable list in the other dataset and try to find the same variables that are already plotted on the screen. If it finds them it will make a comparison plot.
This means that if Tecplot RS is not making a comparison plot, you need to check the variables shown on the plot and make sure that these variables exist in both files.
Please note that to Tecplot RS, a DATE variable is different than a TIME variable, and Tecplot RS does not automatically convert between the two.