A Nyquist plot is always symmetric around the real axis. By default, the negative frequencies are not shown in a Nyquist plot; however, you can control their display using the Negative Frequencies in Nyquist Plot option, in the Analyze > Preference dialog box. If you activate this option, the plot spans all four quadrants and will have symmetry about the real axis. When you de-activate this option, the Nyquist plot will be shown in quadrants 1 and 2 only.