def intercept(axis=:y)
	    case axis
		when :x
		    vertical? ? point.x : (horizontal? ? nil : (slope * point.x - point.y))
		when :y
		    vertical? ? nil : (horizontal? ? point.y : (point.y - slope * point.x))
	    end
	end
def intercept(axis=:y)
	    case axis
		when :x
		    vertical? ? point.x : (horizontal? ? nil : (slope * point.x - point.y))
		when :y
		    vertical? ? nil : (horizontal? ? point.y : (point.y - slope * point.x))
	    end
	end
Find the requested axis intercept
 @param axis	[Symbol]    the axis to intercept (either :x or :y)
 @return [Number]  the location of the intercept