set zero <expression> show zero
gnuplot will not plot a point if its imaginary part is greater in magnitude
than the zero threshold. This threshold is also used in various other
parts of gnuplot as a (crude) numerical-error threshold. The default
zero value is 1e-8. zero values larger than 1e-3 (the reciprocal of the
number of pixels in a typical bitmap display) should probably be avoided, but
it is not unreasonable to set zero to 0.0.