Thanks for the replies, they worked.
It is disappointing that there is not an efficient algorithm to compute such integrals. The integrals are oscillating, all right, but they are damped by gaussian functions rather fast. 

Actually, Maple does not return undefined for the values it does (did) not plot.
Maybe the plot function gives a finite time to calculate the value at each point, and if the function returns no value by then it returns an undefined value?