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Showing content from https://github.com/qutip/qutip/issues/2400 below:

Bug in the rendering of the process matrix · Issue #2400 · qutip/qutip · GitHub

The rendering of the process matrix looks weird in qutip-v5, The bottom of the bars behind is on top of the lower bars in front of it. (This is a CNOT gate with some errors)

Maybe the order of the bar rendering is reversed during the last refactoring of the plotting library?

import numpy as np
import qutip

# Create a noisy CNOT gate
IZ = qutip.tensor([qutip.qeye(2), qutip.sigmaz()])
gate = qutip.gates.cnot()
noisy_gate = (-2.j * np.pi*(IZ/8/20)).expm() @ gate
fid = qutip.average_gate_fidelity(noisy_gate, qutip.gates.cnot())
print(f"noisy CNOT fidelity {fid}")

# Plot process matrix
op_basis = [[qutip.qeye(2), qutip.sigmax(), qutip.sigmay(), qutip.sigmaz()]] * 2
chi = qutip.qpt(qutip.to_super(noisy_gate), op_basis)
qutip.qpt_plot_combined(
    chi, lbls_list=[["i", "x", "y", "z"]] * 2, title="Process Matrix"
)

noisy CNOT fidelity 0.9987669334932512

The bars should be rendered correctly.

QuTiP Version:      5.0.1
Numpy Version:      1.26.4
Scipy Version:      1.12.0
Cython Version:     None
Matplotlib Version: 3.8.3
Python Version:     3.11.8
Number of CPUs:     40
BLAS Info:          Generic
INTEL MKL Ext:      False
Platform Info:      Linux (x86_64)

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