Negative radius
In the polar equation r = f (θ), the radius r can sometimes be negative. In that case, we can preceed in two ways:
There is no such a point;
Take (r, θ), r < 0, to be (-r, θ + π).
GRAPES adopts the second way by default. But you can modify the setting as necessary. To modify it, on the Data panel [Option | Graph Options] or in the [Option] on the [Graph] palette, set [Not Allowed] at [Negative Radius].