Minor improvements to PL plot style (#4690)

**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.

---------

Co-authored-by: Korbinian Kottmann <43949391+Qottmann@users.noreply.github.com>
Co-authored-by: Tom Bromley <49409390+trbromley@users.noreply.github.com>
Co-authored-by: Mudit Pandey <mudit.pandey@xanadu.ai>

doc/code/qml_drawer.rst

@@ -26,7 +26,9 @@ be reset with ``qml.drawer.use_style('black_white')``.
     available_styles
     use_style
 
-The following images are generated via the code:
+The following images are generated via the following code, where ``style`` is
+replaced by an available style specification string (e.g., ``"pennylane"`` or
+``"solarized_light"``):
 
 .. code-block:: python
 
@@ -42,6 +44,12 @@ The following images are generated via the code:
     qml.drawer.use_style(style)
     fig, ax = qml.draw_mpl(circuit)(1.2345, 1.2345)
 
+Note that the shown ``pennylane`` style can be replicated exactly by having the
+Quicksand Bold font installed and refreshing the Matplotlib font cache (by
+deleting the ``fontlist`` file in the Matplotlib cache directory) — to find the
+cache directory, run ``matplotlib.get_cachedir()``. If this font is not
+available, the drawer will fall back on a default font.
+
 Currently Available Styles
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 .. |bws| image:: ../_static/style/black_white_style.png

doc/releases/changelog-dev.md

@@ -259,6 +259,10 @@
   of the computed qubit operator, if imaginary components are smaller than a threshold. 
   [(#4639)](https://github.com/PennyLaneAI/pennylane/pull/4639)
 
+* 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.
+  [(#4690)](https://github.com/PennyLaneAI/pennylane/pull/4690)
+
 * Updated `qml.device`, `devices.preprocessing` and the `tape_expand.set_decomposition` context 
   manager to bring `DefaultQubit2` to feature parity with `default.qubit.legacy` with regards to 
   using custom decompositions. The `DefaultQubit2` device can now be included in a `set_decomposition` 
@@ -408,7 +412,13 @@
   Note that these functions are unstable while device upgrades are underway.
   [(#4555)](https://github.com/PennyLaneAI/pennylane/pull/4555)
 
-* Minor documentation improvement to the usage example in the `qml.QuantumMonteCarlo` page. Integral was missing the differential dx with respect to which the integration is being performed. [(#4593)](https://github.com/PennyLaneAI/pennylane/pull/4593)  
+* Minor documentation improvement to the usage example in the `qml.QuantumMonteCarlo` page.
+  Integral was missing the differential dx with respect to which the integration is being performed.
+  [(#4593)](https://github.com/PennyLaneAI/pennylane/pull/4593)  
+
+* Minor documentation improvement for the use of the `pennylane` style of `qml.drawer` and the
+  `pennylane.drawer.plot` style of `matplotlib.pyplot`. The use of the default font was clarified.
+  [(#4690)](https://github.com/PennyLaneAI/pennylane/pull/4690)
 
 <h3>Bug fixes 🐛</h3>
 
@@ -451,7 +461,8 @@ Stepan Fomichev,
 Joana Fraxanet,
 Diego Guala,
 Soran Jahangiri,
-Korbinian Kottmann
+Korbinian Kottmann,
+Ivana Kurecic,
 Christina Lee,
 Lillian M. A. Frederiksen,
 Vincent Michaud-Rioux,

pennylane/drawer/plot.mplstyle

@@ -1,4 +1,5 @@
 # PennyLane plot palette
+# To use the Quicksand Bold font, first install it and then refresh the Matplotlib font cache (delete the fontlist file in the cache directory). To find the cache directory location, run matplotlib.get_cachedir()
 axes.prop_cycle: cycler('color', ['70CEFF', 'FF87EB', 'FFE096', 'D7A2F6', 'FDC9DE', 'E1EBA8', 'C6FDD9', 'B5F2ED'])
 
 patch.linewidth: 0.5
@@ -11,7 +12,7 @@ axes.edgecolor: BCBABA
 axes.linewidth: 1
 axes.grid: True
 axes.titlesize: x-large
-axes.labelcolor: BCBABA
+axes.labelcolor: black
 
 xtick.color: black
 xtick.direction: out
@@ -22,4 +23,6 @@ grid.color: F0EEEF
 grid.linestyle: :
 
 font.sans-serif: Quicksand, DejaVu Sans, Bitstream Vera Sans, Computer Modern Sans Serif, Lucida Grande, Verdana, Geneva, Lucid, Arial, Helvetica, Avant Garde, sans-serif
-font.weight: bold
+font.weight: bold
+
+figure.dpi: 300