This example demonstrates the implementation of a custom `.BoxStyle`.
Custom `.ConnectionStyle`\s and `.ArrowStyle`\s can be similarly defined.

from matplotlib.patches import BoxStyle
from matplotlib.path import Path
import matplotlib.pyplot as plt


###############################################################################
# Custom box styles can be implemented as a function that takes arguments
# specifying both a rectangular box and the amount of "mutation", and
# returns the "mutated" path.  The specific signature is the one of
# ``custom_box_style`` below.
#
# Here, we return a new path which adds an "arrow" shape on the left of the
# box.
#
# The custom box style can then be used by passing
# ``bbox=dict(boxstyle=custom_box_style, ...)`` to `.Axes.text`.


def custom_box_style(x0, y0, width, height, mutation_size):
    """
    Given the location and size of the box, return the path of the box around
    it.
    Rotation is automatically taken care of.
    Parameters
    ----------
    x0, y0, width, height : float
        Box location and size.
    mutation_size : float
        Mutation reference scale, typically the text font size.
    """
    # padding
    mypad = 0.3
    pad = mutation_size * mypad
    # width and height with padding added.
    width = width + 2 * pad
    height = height + 2 * pad
    # boundary of the padded box
    x0, y0 = x0 - pad, y0 - pad
    x1, y1 = x0 + width, y0 + height
    # return the new path
    return Path([(x0, y0),
                 (x1, y0), (x1, y1), (x0, y1),
                 (x0-pad, (y0+y1)/2), (x0, y0),
                 (x0, y0)],
                closed=True)


fig, ax = plt.subplots(figsize=(3, 3))
ax.text(0.5, 0.5, "Test", size=30, va="center", ha="center", rotation=30,
        bbox=dict(boxstyle=custom_box_style, alpha=0.2))


###############################################################################
# Likewise, custom box styles can be implemented as classes that implement
# ``__call__``.
#
# The classes can then be registered into the ``BoxStyle._style_list`` dict,
# which allows specifying the box style as a string,
# ``bbox=dict(boxstyle="registered_name,param=value,...", ...)``.
# Note that this registration relies on internal APIs and is therefore not
# officially supported.


class MyStyle:
    """A simple box."""

    def __init__(self, pad=0.3):
        """
        The arguments must be floats and have default values.
        Parameters
        ----------
        pad : float
            amount of padding
        """
        self.pad = pad
        super().__init__()

    def __call__(self, x0, y0, width, height, mutation_size):
        """
        Given the location and size of the box, return the path of the box
        around it.
        Rotation is automatically taken care of.
        Parameters
        ----------
        x0, y0, width, height : float
            Box location and size.
        mutation_size : float
            Reference scale for the mutation, typically the text font size.
        """
        # padding
        pad = mutation_size * self.pad
        # width and height with padding added
        width = width + 2.*pad
        height = height + 2.*pad
        # boundary of the padded box
        x0, y0 = x0 - pad, y0 - pad
        x1, y1 = x0 + width, y0 + height
        # return the new path
        return Path([(x0, y0),
                     (x1, y0), (x1, y1), (x0, y1),
                     (x0-pad, (y0+y1)/2.), (x0, y0),
                     (x0, y0)],
                    closed=True)


BoxStyle._style_list["angled"] = MyStyle  # Register the custom style.

fig, ax = plt.subplots(figsize=(3, 3))
ax.text(0.5, 0.5, "Test", size=30, va="center", ha="center", rotation=30,
        bbox=dict(boxstyle="angled,pad=0.5", alpha=0.2))

del BoxStyle._style_list["angled"]  # Unregister it.

plt.show()

keywords: pythonmatplotlib