Custom box styles

This example demonstrates the implementation of a custom BoxStyle. Custom ConnectionStyles and ArrowStyles 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, mutation_aspect=1):
    """
    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.
    mutation_aspect
        Mutation aspect ratio.
    """
    # We ignore mutation_aspect. This is okay in general.

    # 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))

Out:

Text(0.5, 0.5, 'Test')

Alternatively, custom box styles can be implemented as subclasses of matplotlib.patches.BoxStyle._Base, by overriding the transmute method, as demonstrated below.

The subclass 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 approach relies on internal APIs and is therefore not officially supported.

class MyStyle(BoxStyle._Base):
    """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 transmute(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.

        Notes
        -----
        Unlike when defining the box style as a function (as in
        `custom_box_style`), here there is no *mutation_aspect* parameter.
        Matplotlib will first squeeze the box's y-axis by *mutation_aspect*
        before calling the `transmute` method, and then later reexpand the
        y-axis by the same amount.
        """
        # 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()