pennylane style is non-xkcd, pennylane_sketch is squiggly (#4880)

**Context:**
The PennyLane MPL-drawer style is great, but some users would prefer
smoother plots for more professional contexts, eg. publications. We
should continue to offer the xkcd-like style, but perhaps "pennylane"
should be the non-xkcd style. I propose we make this change, and add
"pennylane_sketch" to keep the squiggly, existing version.

**Description of the Change:**
- Rename the "pennylane" style to "pennylane_sketch"
- Introduce the "pennylane" style to be identical to the old style, but
with non-sketch lines

**Benefits:**
A clean "pennylane" style for professional-looking data plotting, plus
the sketch style is still offered.

**Possible Drawbacks:**
Would be breaking for the "pennylane" style. We can do a deprecation
cycle if people prefer that - it's easy enough to add a warning, just
lmk. I'd prefer not to add the warning, because then people will use the
"pennylane" style that we recommend and get a warning in their face!

[sc-50495]

doc/releases/changelog-dev.md

@@ -145,6 +145,10 @@
 * The `rot` decomposition now has support for returning a global phase.
   [(#4869)](https://github.com/PennyLaneAI/pennylane/pull/4869)
 
+* The `"pennylane_sketch"` MPL-drawer style has been added. This is the same as the `"pennylane"`
+  style, but with sketch-style lines.
+  [(#4880)](https://github.com/PennyLaneAI/pennylane/pull/4880)
+
 <h3>Breaking changes 💔</h3>
 
 * The decomposition of `GroverOperator` now has an additional global phase operation.
@@ -171,6 +175,9 @@
   because it depended on the now-deprecated `Observable.return_type` property.
   [(#4762)](https://github.com/PennyLaneAI/pennylane/pull/4762)
 
+* The `"pennylane"` MPL-drawer style now draws straight lines instead of sketch-style lines.
+  [(#4880)](https://github.com/PennyLaneAI/pennylane/pull/4880)
+
 <h3>Deprecations 👋</h3>
 
 * `single_tape_transform`, `batch_transform`, `qfunc_transform`, and `op_transform` are deprecated.

pennylane/drawer/draw.py

@@ -335,11 +335,11 @@ def draw_mpl(
         show_all_wires (bool): If True, all wires, including empty wires, are printed.
         decimals (int): How many decimal points to include when formatting operation parameters.
             Default ``None`` will omit parameters from operation labels.
-        style (str): visual style of plot. Valid strings are ``{'black_white', 'black_white_dark', 'sketch', 'pennylane',
-            'sketch_dark', 'solarized_light', 'solarized_dark', 'default'}``. If no style is specified, the
-            global style set with :func:`~.use_style` will be used, and the initial default is 'black_white'.
-            If you would like to use your environment's current rcParams, set `style` to "rcParams".
-            Setting style does not modify matplotlib global plotting settings.
+        style (str): visual style of plot. Valid strings are ``{'black_white', 'black_white_dark', 'sketch',
+            'pennylane', 'pennylane_sketch', 'sketch_dark', 'solarized_light', 'solarized_dark', 'default'}``.
+            If no style is specified, the global style set with :func:`~.use_style` will be used, and the
+            initial default is 'black_white'. If you would like to use your environment's current rcParams,
+            set ``style`` to "rcParams". Setting style does not modify matplotlib global plotting settings.
         fontsize (float or str): fontsize for text. Valid strings are
             ``{'xx-small', 'x-small', 'small', 'medium', large', 'x-large', 'xx-large'}``.
             Default is ``14``.

pennylane/drawer/style.py

@@ -151,6 +151,15 @@ def _pennylane():
     if "Quicksand" in {font.name for font in fm.FontManager().ttflist}:  # pragma: no cover
         plt.rcParams["font.family"] = "Quicksand"
     plt.rcParams["font.weight"] = "bold"
+    plt.rcParams["path.sketch"] = None
+
+
+@_needs_mpl
+def _pennylane_sketch():
+    """Apply the PennyLane-Sketch style to matplotlib's configuration. This function
+    modifies ``plt.rcParams``.
+    """
+    _pennylane()
     plt.rcParams["path.sketch"] = (1, 250, 1)
 
 
@@ -178,6 +187,7 @@ def _sketch_dark():
     "black_white_dark": _black_white_dark,
     "sketch": _sketch,
     "pennylane": _pennylane,
+    "pennylane_sketch": _pennylane_sketch,
     "sketch_dark": _sketch_dark,
     "solarized_light": _solarized_light,
     "solarized_dark": _solarized_dark,
@@ -209,6 +219,7 @@ def use_style(style: str):
     * ``'black_white_dark'``
     * ``'sketch'``
     * ``'pennylane'``
+    * ``'pennylane_sketch'``
     * ``'sketch_dark'``
     * ``'solarized_light'``
     * ``'solarized_dark'``

tests/drawer/test_style.py

@@ -37,6 +37,7 @@ def test_available_styles():
         "black_white_dark",
         "sketch",
         "pennylane",
+        "pennylane_sketch",
         "sketch_dark",
         "solarized_light",
         "solarized_dark",
@@ -95,10 +96,11 @@ def test_sketch_style():
     assert plt.rcParams["path.sketch"] == (1, 100, 2)
 
 
-def test_pennylane_style():
+@pytest.mark.parametrize("style,sketch", [("pennylane", None), ("pennylane_sketch", (1, 250, 1))])
+def test_pennylane_style(style, sketch):
     """Tests the pennylane style sets ``plt.rcParams`` with correct values"""
 
-    _set_style("pennylane")
+    _set_style(style)
 
     almost_black = "#151515"  # less harsh than full black
     assert plt.rcParams["figure.facecolor"] == "white"
@@ -111,7 +113,7 @@ def test_pennylane_style():
     assert plt.rcParams["lines.color"] == "black"
     assert plt.rcParams["text.color"] == "black"
     assert plt.rcParams["font.weight"] == "bold"
-    assert plt.rcParams["path.sketch"] == (1, 250, 1)
+    assert plt.rcParams["path.sketch"] == sketch
 
 
 def test_sketch_style_dark():