Adding data to previously created plots.

I'd like to add an additional line of data to an already created plot that has been saved as a .fig (I'm open to saving it in a different format if need be). From what I can tell, the only way to do this is to open the previously created plot, pull the data from that plot, and use it to recreate a new plot. It seems very redundant to do so and recreate all of the titles/text/formatting aspects of the original plot. Does anyone know of a way around this?