Fix error in plot_state_city that negative value in real is not displayed

[mi2valley]

Summary

Fix an error that the negative value is not displayed when a state with a real negative value is used as input for the plot_state_city method.

Details and comments

A simple example that shows the original problem:

from qiskit import QuantumCircuit, transpile
from qiskit.tools.jupyter import *
from qiskit.visualization import *
from ibm_quantum_widgets import *
from qiskit_ibm_runtime import QiskitRuntimeService, Sampler, Estimator, Session, Options
from qiskit.quantum_info import DensityMatrix

service = QiskitRuntimeService(channel="ibm_quantum")

circ = QuantumCircuit(2)

circ.x(0)
circ.h(0)
circ.cx(0, 1)

dm = DensityMatrix.from_instruction(circ)

plot_state_city(dm, title='Density Matrix')

After fixing plot_state_city method:

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[AlexanderGroeger]

This might already be fixed in my PR, but it is still waiting for review.

[mi2valley]

This might already be fixed in my PR, but it is still waiting for review.

I tried plot_state_city in your PR, but negative values in real did not show up as bars.

Could you please check if the bars for negative values are displayed as shown in the second density matrix figure attached?
I think we are fixing another issue.

[AlexanderGroeger]

Could you please check if the bars for negative values are displayed as shown in the second density matrix figure attached? I think we are fixing another issue.

Yep. It looks like this is a separate issue.
I have removed the line causing this issue in my PR, and it yields the figure you have presented.

[1ucian0]

This PR seems supersede by #10590, so closing for now. Thanks!