First of all, qutip is not a visualisation library, even though it does provide some visualisation functionalities, mostly leveraging matplotlib.
However, because qutip does provide handy functionalities to plot Bloch spheres and points on it, it does make sense to ask how one can tweak such functionalities to for example add tangent vectors to the bloch ...
So, this can happen when the ode solvers automatic step size solver thinks that nothing is going on and takes a big step over the pulse. There is a max_step option that you can set. In practice, setting this to something like half the width of the smallest pulse in a pulse sequence works quite well. Then the solver sees the pulses and adjusts accordingly.
I found a solution to my problem here https://github.com/qutip/qutip/issues/1027
Apparently plotting points on the Bloch sphere does not work with Matplotlib v3.1.0.
So I had to downgrade to v.3.0.3 for it to work!
That is a very old paper, corresponding to version one of the software. It is now on version 4.x. It is best to see the current documentation for how to use the Bloch sphere: http://qutip.org/docs/latest/guide/guide-bloch.html
Mixed states are represented by density matrices cf. Density matrices for pure states and mixed states. This is the QuTiP documentation for density matrices; check the 4th example. The ket2dm function takes input a ket or bra vector and returns density matrix formed by the outer product. So say for a 50-50 mixture the following code should work:
from qutip ...