^63 : 2^63] for 64-bit integers, the result is returned as a floating
point value instead. This is not treated as an error.
Example:
gnuplot> set overflow float
gnuplot> A = 2**62 - 1; print A, A+A, A+A+A
4611686018427387903 9223372036854775806 1.38350580552822e+19