but why does its derivative function has to be absolutely true?
have you checked that this function complies with the rules of differentiability at x=0? [i'm pretty sure vertical tangents are exceptions to differentiation, and this is marked by its derivative's domain. this, however, does not imply it doesn't have a gradient]
if you differentiate this with vector calculus, i'm sure you'll end up with a unit tangent of

doing vector calculus now [Captain: dont laugh, haha]
the relationship definining this function is continuous, and can be described by the relationship:


where t is a parameter.
this relationship can then be defined as a space-curve relative to O as:
 = xi+yj = t^3\cdot i + t \cdot j)
 = 3t^2 \cdot i + j)
hence its unit tangent can be described as:
 = \frac{\bar{r}'(t)}{|\bar{r}'(t)|} = \frac{3t^2\cdot i + j}{\sqrt{9t^4+1}})
 = \frac{0\cdot i + j}{\sqrt{0+1}} = j)
yay vertical tangent =]