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Please note that the 2D radiation pattern is cut plane view of the 3D radiation pattern by fixing the value of phi or theta. In your case, theta is varied from -180 to 180 for a fixed phi value. For better understanding, you can overlay these radiation patterns (for a single phi or theta value) in the 3D modeler window on the antenna geometry. See the below image for the steps.
You can also change the scale and transparency of these plots using the following steps.