Mastering Matplotlib Anchored Objects

Introduction

In this lab, you will learn how to use Anchored Objects in Matplotlib. Anchored Objects are used to add auxiliary objects to a plot. These objects can be used to add annotations, scale bars, and legends to a plot.

VM Tips

After the VM startup is done, click the top left corner to switch to the Notebook tab to access Jupyter Notebook for practice.

Sometimes, you may need to wait a few seconds for Jupyter Notebook to finish loading. The validation of operations cannot be automated because of limitations in Jupyter Notebook.

If you face issues during learning, feel free to ask Labby. Provide feedback after the session, and we will promptly resolve the problem for you.

Import Libraries

The first step is to import the required libraries. We will be using Matplotlib for this lab.

from matplotlib import pyplot as plt
from matplotlib.lines import Line2D
from matplotlib.offsetbox import (AnchoredOffsetbox, AuxTransformBox,
                                  DrawingArea, TextArea, VPacker)
from matplotlib.patches import Circle, Ellipse

Create a Figure

The next step is to create a figure. We will create a simple figure with a single subplot.

fig, ax = plt.subplots()
ax.set_aspect(1)

Add Anchored Text

In this step, we will add a text box anchored to the upper-left corner of the figure.

def draw_text(ax):
    """Draw a text-box anchored to the upper-left corner of the figure."""
    box = AnchoredOffsetbox(child=TextArea("Figure 1a"),
                            loc="upper left", frameon=True)
    box.patch.set_boxstyle("round,pad=0.,rounding_size=0.2")
    ax.add_artist(box)

draw_text(ax)

Add Anchored Circles

In this step, we will add two circles to the plot using Anchored Objects.

def draw_circles(ax):
    """Draw circles in axes coordinates."""
    area = DrawingArea(width=40, height=20)
    area.add_artist(Circle((10, 10), 10, fc="tab:blue"))
    area.add_artist(Circle((30, 10), 5, fc="tab:red"))
    box = AnchoredOffsetbox(
        child=area, loc="upper right", pad=0, frameon=False)
    ax.add_artist(box)

draw_circles(ax)

Add Anchored Ellipse

In this step, we will add an ellipse to the plot using Anchored Objects.

def draw_ellipse(ax):
    """Draw an ellipse of width=0.1, height=0.15 in data coordinates."""
    aux_tr_box = AuxTransformBox(ax.transData)
    aux_tr_box.add_artist(Ellipse((0, 0), width=0.1, height=0.15))
    box = AnchoredOffsetbox(child=aux_tr_box, loc="lower left", frameon=True)
    ax.add_artist(box)

draw_ellipse(ax)

Add Size Bar

In this step, we will add a size bar to the plot using Anchored Objects.

def draw_sizebar(ax):
    """
    Draw a horizontal bar with length of 0.1 in data coordinates,
    with a fixed label center-aligned underneath.
    """
    size = 0.1
    text = r"1$^{\prime}$"
    sizebar = AuxTransformBox(ax.transData)
    sizebar.add_artist(Line2D([0, size], [0, 0], color="black"))
    text = TextArea(text)
    packer = VPacker(
        children=[sizebar, text], align="center", sep=5)  ## separation in points.
    ax.add_artist(AnchoredOffsetbox(
        child=packer, loc="lower center", frameon=False,
        pad=0.1, borderpad=0.5))  ## paddings relative to the legend fontsize.

draw_sizebar(ax)

Display the Plot

The final step is to display the plot.

plt.show()

Summary

In this lab, you learned how to use Anchored Objects in Matplotlib. You learned how to add text, circles, ellipses, and size bars to a plot using Anchored Objects. Anchored Objects are a powerful tool that can be used to add annotations and legends to a plot.

Matplotlib Anchored Objects