Thanks for the tip on using Table. It seems that Plot uses only Reals when generating data for the graph. And that seems especially true when one explicitly sets the increment value to .1. I considered this when the plot came up empty. But I didn't know how to tell Mathematica to look for all possible Rationals in this domain--there are bound to be an infinite number as well as an infinite number of non-rationals. (This is part of a textbook problem from an old Calculus book. I wonder what "they" expected the student to come up with.)

As you have kindly pointed out (here, and in an earlier response to my post that you must have edited out), I have misused Element. (I'm still struggling to grasp how to implement conditionals versus assertions.) Thanks for any help my understanding.