Minor improvements to PL plot style

[ikurecic]

Context: Documentation for the use of the pennylane style of qml.drawer and the pennylane.drawer.plot style of matplotlib.pyplot with regards to the default (Quicksand Bold) font needed to be improved, and minor cosmetic changes to plots were suggested.

Description of the Change: Plots generated with the pennylane.drawer.plot style of matplotlib.pyplot now have black axis labels and are generated at a default DPI of 300. Documentation for the use of the pennylane style of qml.drawer and the pennylane.drawer.plot style of matplotlib.pyplot has also been improved.

Benefits: Looks better. 😁

Possible Drawbacks: None.

Related GitHub Issues: None.

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[Qottmann]

Very nice @ikurecic 🥂
some minor comments and I noticed that the output for me is slightly different, not sure if this is expected or not

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5f246a9) 99.64% compared to head (4870ed2) 99.63%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4690      +/-   ##
==========================================
- Coverage   99.64%   99.63%   -0.01%     
==========================================
  Files         377      377              
  Lines       33999    33738     -261     
==========================================
- Hits        33878    33616     -262     
- Misses        121      122       +1     

see 42 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ikurecic]

[sc-48246]

[trbromley]

One issue with using Quicksand Bold as the font is that PhaseShift does not draw correctly (for me at least):

import pennylane as qml

dev = qml.device("default.qubit")

@qml.qnode(dev)
def f():
    qml.PhaseShift(0.4, wires=0)
    return qml.state()

qml.draw_mpl(f, style="pennylane")()

UserWarning: Glyph 981 (\N{GREEK PHI SYMBOL}) missing from current font.

Not sure how we can fix this though other than not using that font.

[ikurecic]

@trbromley Yikes! 🤦‍♀️ We do have 'another' PL font, but that's still being perfected — I'll make sure to include this in the considerations there. Thank you!