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Using Sage plotting capability on data from PARI/GP (1)

This is a short follow-up question from this one.

I would like to produce an implicit plot (a contour plot where the real or imaginary values of a function are zero):

implicit_plot(lambda x,y:g(x+y*I),(x,-3,3),(y,-3,3))

This works fine, however I want to use GP/Pari to evaluate the zeta function and therefore wrote:

implicit_plot(lambda x,y:real_part(g(x+y*I)),(x,-3,3),(y,-3,3))

but then keep getting an error message:

PARI/GP ERROR: at top-level: sage[45020]=sage[16][1] ^--- incorrect type in _[_] OCcompo1 [not a vector] (t_REAL).*

For 'normal' plots like this one:

plot(lambda x:(g(x)),(x,3,6))

the interface with GP works fine, so I probably do something wrong using multiple variables or complex numbers? It doesn't seem to be zeta-function specific (like the pole at s=1), since it also fails for e.g. the cos-function.

Grateful for any advice on how to make this work.