Plot State City Visualization Fixes

[AlexanderGroeger]

This is my first PR, please correct me as needed. :)

Summary

The qiskit.visualization module has a function plot_state_city which yields a matplotlib 3d bar chart representing the density matrix of a quantum state.

Fixes #8413
Fixes #3315
Related to #10862

Details and comments

Note for some extreme choices of figsize, the plot labels can still be cut off. The primary issue is matplotlib cannot use tight_layout because there's a lack of space.

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[AlexanderGroeger]

Looks like the qiskit-terra test failed on Run image test, specifically test_plot_state_city did not succeed. Apparently the error was AssertionError: 0.930396 not greater than or equal to 0.99 coming from

ratio = VisualTestUtilities._save_diff(
            self._image_path(fname),
            self._reference_path(fname),
            fname,
            FAILURE_DIFF_DIR,
            FAILURE_PREFIX,
        )
        self.assertGreaterEqual(ratio, 0.99)

[Cryoris]

Since your PR changes the output of the plot_state_city function, you probably have to change the reference pictures in used in the tests. See e.g. the comments of #10619 on how to do this 🙂

[coveralls]

Pull Request Test Coverage Report for Build 5799800653

• 0 of 59 (0.0%) changed or added relevant lines in 1 file are covered.
• 25 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.01%) to 87.256%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/visualization/state_visualization.py	0	59	0.0%

Files with Coverage Reduction	New Missed Lines	%
qiskit/extensions/quantum_initializer/squ.py	2	80.0%
qiskit/visualization/state_visualization.py	2	58.72%
crates/qasm2/src/lex.rs	3	90.63%
crates/qasm2/src/parse.rs	18	96.67%

Totals
Change from base Build 5787606820:	0.01%
Covered Lines:	74240
Relevant Lines:	85083

---

💛 - Coveralls

[AlexanderGroeger]

I believe this PR should be ready for review, sorry about the multiple merges; I wasn't sure if I should keep my PR up-to-date or not.

[AlexanderGroeger]

If this PR could be reviewed and closed soon, that would be great. Thanks!

[1ucian0]

Thanks a lot for your contribution. Original, the bug in #10862 was introduced by a rounding error that made testing. Do you know if your changes are sensitive to them?

I never understood why there is a color difference for 0.0:

The RGB values shouldn't be the same?

[AlexanderGroeger]

Regarding a rounding error that made testing, I removed this line altogether without using any substitute code. I didn't quite understand why this was an issue. It sounded like bars with zero height were considered negative in the visualization, but I'm wondering if this was just a problem with how the dividing zero plane was plotted in which that has been corrected.
The reason for the color difference eludes me too. I was initially considering the issue was with generate_facecolors, but there is nothing obvious there. I'm starting to wonder if it related to how matplotlib handles 3d projections.

[1ucian0]

Is this fixing #8413 , #3315 and #10862 ? If so uses Fixes #<number> in the main message for close them automatically once this gets merged.

[AlexanderGroeger]

Is this fixing #8413 , #3315 and #10862 ? If so uses Fixes #<number> in the main message for close them automatically once this gets merged.

Yes. I edited the PR description to reflect this. I'm still a bit new to some of these expectations, so thank you!

[1ucian0]

Take a look to the black issue.

Dropping the label for the axes (Re and Im) was intentional?

[AlexanderGroeger]

Yes. I figured putting subtitles over each subplot would be easier to read instead. The primarily reason why the y-axis labels were moved was because of spacing issues related to #8413.

[1ucian0]

Thanks!