I'm not sure I fully understand your question.
If you're asking what the checkbox labeled "Show smooth curve" does, you can see that by viewing the sufunction named updateSmoothLine in the file for mairplot.
Or maybe you are asking whether there's a command-line option that creates this smooth curve (so that you don't have to click on the checkbox). If that's what you're asking, I think there is no easy way to do that. But once again, you can always view the code for udpateSmoothLine. Perhaps you could use that code as inspiration and write something similar that does exactly what you want.