def intercept(axis=:y)
	    case axis
		when :x
		    vertical? ? first.x : (horizontal? ? nil : (first.x - first.y/slope))
		when :y
		    vertical? ? nil : (horizontal? ? first.y : (first.y - slope * first.x))
	    end
	end
def intercept(axis=:y)
	    case axis
		when :x
		    vertical? ? first.x : (horizontal? ? nil : (first.x - first.y/slope))
		when :y
		    vertical? ? nil : (horizontal? ? first.y : (first.y - slope * first.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