void set_curve_type(GtkCurveType type);

Use this method when you want to set the type of the curve that you want the user to draw. The existing curve will remain unchanged except when changing from a free curve to a linear or spline curve, in which case, the curve will be changed as little as possible.

Pass 0 as a parameter if you want the curve type to be linear, 1 for a spline curve and 2 for a free type curve.