This is a good question. Please first refer this article:
https://support.lumerical.com/hc/en-us/articles/360034394294-Understanding-direction-unit-vector-coordinates-in-far-field-projections
Usually we have a plane monitor, so the far field is only on half sphere, thus theta is from 0 to 90, 2pi space.
The far field graph does not label theta larger than 90, due to visual betterness.
90 to 180 will be the backward propagation, which is very small due to PML reflection and source injection error.
in short, a plane monitor can only give its farfield in half sphere. and we cannot compress the sphere like world map to view the farfield in 4pi space, as it introduces distortion.
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