Repository: Qiskit/qiskit

Issue:
Alternate example for plot_bloch_multivector
### Environment

- **Qiskit Terra version**: 0.19.1




### What is happening?

The example for the plot_bloch_multivector doc looks broken - https://qiskit.org/documentation/stubs/qiskit.visualization.plot_bloch_multivector.html#qiskit.visualization.plot_bloch_multivector

The example is trying to visualize a bell state and the entanglement isn't clearly visualizable with bloch sphere and hence there are 2 blank bloch spheres that may not depict the idea of the function clearly!


![Screenshot from 2021-12-16 15-38-27](https://user-images.githubusercontent.com/61380731/146354927-ada94920-20d9-46da-91fc-98e4f4d8336e.png)



### How can we reproduce the issue?

The issue can be checked out here! https://qiskit.org/documentation/stubs/qiskit.visualization.plot_bloch_multivector.html#qiskit.visualization.plot_bloch_multivector

LIne in docs - 
https://github.com/Qiskit/qiskit-terra/blob/4301fc47c73507e489dbafc7bae490ab29736daa/qiskit/visualization/state_visualization.py#L263-L277

### What should happen?

An example that clearly depicts a use case for the `plot_bloch_multivector` for people checking out the documentation

### Any suggestions?

Since the examples are uniform across all the sections of https://qiskit.org/documentation/apidoc/visualization.html#counts-and-state-visualizations, contributor may have to take into consideration that if at all this example is to be modified, the subsection docs `plot_histogram` , `plot_state_city`, `plot_state_hinton`, `plot_state_paulivec` and `plot_state_qsphere `may have to be similarly accommodated since currently all seem to be using the same example across to depict the visualisation ideas uniformly in their respective sections.


Discussion:
Actually the entanglement is "visible" in that plot because it is tracing over qubits and giving the maximally mixed state which is just a point at the origin of the sphere.  That is to say it is showing what it is supposed to show in that example.
> Actually the entanglement is "visible" in that plot because it is tracing over qubits and giving the maximally mixed state which is just a point at the origin of the sphere. That is to say it is showing what it is supposed to show in that example.

Totally agreed! :smile: It may not be something that would immediately strike as an example use case for new qiskitters referring to the example in my opinion. A state which has a clear visible plot could be much quicker for a viewer to get an idea as to what the function offers! :smile: 

Maybe a modification to add more prominent plot points while plotting single points could also be a suggestion! :grin: 


May I take this issue?
Sure, I'll assign this one to you so people know you're working on it.

Resolving patch:
diff --git a/qiskit/visualization/state_visualization.py b/qiskit/visualization/state_visualization.py
--- a/qiskit/visualization/state_visualization.py
+++ b/qiskit/visualization/state_visualization.py
@@ -270,10 +270,10 @@ def plot_bloch_multivector(
 
             qc = QuantumCircuit(2)
             qc.h(0)
-            qc.cx(0, 1)
+            qc.x(1)
 
             state = Statevector.from_instruction(qc)
-            plot_bloch_multivector(state, title="New Bloch Multivector", reverse_bits=False)
+            plot_bloch_multivector(state)
     """
     if not HAS_MATPLOTLIB:
         raise MissingOptionalLibraryError(


Test patch: