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plot them just like any other number. It just means you have to know how to square fractions and add them.
For instance if
f(x) = 2x^2-3x+7
f(2/3) = 2(4/9) - 3(2/3) + 7
= 8/9 - 2 + 7
= 5 8/9
= 53/9 or 5.88888