Circular, rectangular and log or lin
Unless the 'Plot in dB' checkbox is de-selected, plots are shown as relative power in dB. You can show a plot of relative field strength (linear scale only) by un-checking the 'Plot in dB' checkbox. This makes your beam really look like a beam, but you are still looking at the same antenna, only in a different way.
To alternate between a circular plot and a rectangular plot, click the 'Rectangular' button. The button face then changes to 'Circular'. A rectangular plot is more useful when looking at the side-lobes. You can swap between the two types whenever you wish.
Likewise, you can change the scale of either plot by clicking the 'Lin Scale' button. The button face then changes to 'Log Scale'. Be aware that the values plotted are still relative power in dB unless the 'Plot in dB' checkbox is de-selected, in which case it is relative field strength.
The rectangular plot shows a black circle around the start/end point of the data, and the vertical lines are at 10 degree spacing.
To quickly centre the -3dB points (usually the main lobe) in a rectangular plot, first of all switch to circular display, then click the 'Rescale' button, then switch back to rectangular display. Any desired starting position can be achieved by manually changing the 'Degrees Rotated' value.