The simplest way is to change the values in your plot command. Divide all your fs times by 1000 and now they're picosecond times.
instead of: plot(time_vector , function_of_time)
try: plot(time_vector/1000, function_of_time)
You can also force limits on your x-axis both with the xlim and axis commands. You can set your minor and major tick marks with the set command or the plottools UI. But start with the above.
