Remove hardcoded styling in plot_histogram

[nonhermitian]

Summary

plot_histogram was hardcoding things like bar colors, font and figure sizes, etc, that were preventing Matplotlib style files from being faithfully applied to the images.

Here we remove the hardcoded values, allowing complete customization following the loaded stylelib. Users wishing to modify the default settings further can either make their own styles, or take advantage of the fact that one can pass an Axes object, giving complete control of the customization post-generation.

Details and comments

The bar colors are now sourced from the rcParams which are in turn set by the style. Most hard-coded options have been removed save for the rotation of the x-axis labels that need to be rotated. I have also disabled bar_labels by default as for anything more than 2-3Q, the labels are unreadable, and unnecessary given the y-axis is also labeled and the data is included in the Counts themselves.

This allows one to make figures that look like:

or

easily

[qiskit-bot]

Thank you for opening a new pull request.

Before your PR can be merged it will first need to pass continuous integration tests and be reviewed. Sometimes the review process can be slow, so please be patient.

While you're waiting, please feel free to review other open PRs. While only a subset of people are authorized to approve pull requests for merging, everyone is encouraged to review open pull requests. Doing reviews helps reduce the burden on the core team and helps make the project's code better for everyone.

One or more of the the following people are requested to review this:

• @Qiskit/terra-core
• @enavarro51

[nonhermitian]

This should wait for #8762 to be merged so that I can refactor on top of those changes.

[1ucian0]

Thanks a lot for this PR. The feature makes sense to me. Just a small comment on bar_labels that I dont get.