Adding a PennyLane style for circuits and matplotlib plots

[ikurecic]

Enables use of draw_mpl with style 'pennylane' and matplotlib.pyplot with style 'pennylane.drawer.plot' to create circuit diagrams and plots in the PennyLane style.

Note: The relevant Matplotlib style sheet installs together with PennyLane and can be used independently of PennyLane, but it will not show up in the list of available styles (matplotlib.pyplot.style.available)
Note2: This style looks best with the Quicksand Bold font (if font not seen in plots after installation, delete Matplotlib font cache file), but if the user doesn't have it installed, it will fall back on the Matplotlib default — no errors are expected to spawn
Note3 (for testing): Matplotlib doesn't like pip install -e . and will resist you. Stick to pip install .

• Circuit example:

import pennylane as qml

dev = qml.device('lightning.qubit', wires=(0,1,2,3))

@qml.qnode(dev)
def circuit(x, z):
    qml.QFT(wires=(0,1,2,3))
    qml.IsingXX(1.234, wires=(0,2))
    qml.Toffoli(wires=(0,1,2))
    qml.CSWAP(wires=(0,2,3))
    qml.RX(x, wires=0)
    qml.CRZ(z, wires=(3,0))
    return qml.expval(qml.PauliZ(0))

fig, ax = qml.draw_mpl(circuit, style=’pennylane’)(1,1)
fig.show()

• Plot example:

import matplotlib.pyplot as plt
import numpy as np

plt.style.use('pennylane.drawer.plot')
for i in range(3):
    plt.plot(np.random.rand(10))

[codecov]

Codecov Report

Merging #3950 (bcae2f6) into master (edefd31) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3950   +/-   ##
=======================================
  Coverage   99.71%   99.71%           
=======================================
  Files         378      378           
  Lines       34231    34245   +14     
=======================================
+ Hits        34134    34148   +14     
  Misses         97       97           

Files Changed	Coverage Δ
pennylane/drawer/draw.py	100.00% <ø> (ø)
pennylane/drawer/style.py	99.09% <100.00%> (+0.13%)	⬆️

[ikurecic]

EOW status:

• Plots: PennyLane palette, uses Quicksand Bold font if installed (if not, falls back on sans-serif).
  
• Circuits: Same as plots.
  

@josh146 , do you want me to un-draft this and see about including it? I know a circuit plot redesign is in sights, but Tarik is fine with using this as a starting point. ☺

[josh146]

@ikurecic I think this is a really nice iterative improvement! I'll also tag @trbromley here in terms of, where this could fit into the PL Q2 roadmap

[ikurecic]

Current status:

• Plots: PennyLane palette (main three colors are the brightest blue, mid-level pink, and brightest yellow), uses Quicksand Bold font if present (if not, falls back on sans-serif and shouldn't spawn warnings).
  
• Circuits: Same color scheme as plots, but skips the yellow. (I'm not fully vibing with the color scheme, but I'm not sure we have a great option to incorporate the yellow here…)
  

[timmysilv]

this looks great! I'd like some product people to weigh in on how we announce/inform users about the plot style in general because it's not something we've really done before, but I think it'll be awesome to have. the MPL circuits look great, I'm wondering about how to factor in a third colour as well but we don't really need it. maybe measurements? idk. I do think we shouldn't put the config at the top-level because people probably see it all the time. also do you know where that files appear when someone pip installs PL?

[josh146]

Thanks @ikurecic! Only minor changes on my end.

Circuits: Same color scheme as plots, but skips the yellow. (I'm not fully vibing with the color scheme, but I'm not sure we have a great option to incorporate the yellow here…)

We could have blue for single wire, yellow for multi wire, or vice versa? I also like the idea of using yellow for measurements.

Note: don't forget to also update the docstring of qml.draw_mpl to list pennylane as an available style.

[ikurecic]

Hey @timmysilv and @josh146 , if you're happy with the Matplotlib file path, I would suggest merging this as is now, because adding a PennyLane yellow for measurements in the circuit diagram would take some extra structures (not adjusting the style, but adjusting how styles work) and I don't think I have the time for that right now, unfortunately.

I'd update it the next time there's an empty block in my calendar (i.e., before the next release). :)
Sounds good?

[timmysilv]

super excited to have this in!

[josh146]

Good to go from my end, once the docstring https://xanaduai-pennylane--3950.com.readthedocs.build/en/3950/code/qml_drawer.html is correctly showing the new style :)