5 Best Ways to Put Text at the Corner of an Equal Aspect Figure in Python Matplotlib

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💡 Problem Formulation: When visualizing data using Python’s Matplotlib, there might be a need to annotate figures with text placed at specific corners to provide additional context or information. The aspect ratio of the figure should remain equal to ensure that the geometry of the plot is maintained accurately. The following techniques will demonstrate how to position text at the corners of an equal aspect figure in Python Matplotlib, assuming we want to add the text “Data Point” at the bottom right corner of a square plot.

Method 1: Using text() with Figure Coordinates

This method involves using the text() method combined with the transformation parameter to position the text into the figure coordinates system. These coordinates range from 0 to 1, where (0,0) is the bottom left corner and (1,1) is the top right corner of the figure.

Here’s an example:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.set_aspect('equal')

# Adding text at the bottom right corner
plt.text(0.95, 0.05, 'Data Point', horizontalalignment='right', verticalalignment='bottom', transform=fig.transFigure)

plt.show()

The output will be a plot with ‘Data Point’ annotated at the bottom right.

This technique is straightforward, utilizing figure coordinates to place the text at the specified location within the figure irrespective of the axis scales. This approach ensures the text remains anchored at the corner even when the plot is resized.

Method 2: Using annotate() with Axes Fraction

With the annotate() method, you can use the xycoords parameter set to ‘axes fraction’ to specify the position of the text in terms of axes coordinate system fraction, which is also independent of the actual data range.

Here’s an example:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.set_aspect('equal')

# Annotating text at the bottom right corner using axes fraction
ax.annotate('Data Point', xy=(1, 0), xycoords='axes fraction', xytext=(-10, 10),
             textcoords='offset points', ha='right', va='bottom')

plt.show()

The output will be a plot with ‘Data Point’ placed at the bottom right using the axes fraction coordinates.

This method allows for more complex positions using offsets in points from the specified corner, which can be useful for fine-tuning the text position relative to elements like the axis ticks or labels.

Method 3: Placing Text Relative to Data Coordinates

By passing the positional arguments to text(), you can specify the desired location in data coordinates. However, you need to know the xlim and ylim to position the text at a corner, which can vary with different data sets.

Here’s an example:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.set_aspect('equal')

# Assuming known limits of the data
xlim = ax.get_xlim()
ylim = ax.get_ylim()

# Placing text at the bottom right of the known data limits
ax.text(xlim[1], ylim[0], 'Data Point', horizontalalignment='right', verticalalignment='bottom')

plt.show()

The output will show ‘Data Point’ placed in the bottom right according to the data coordinates.

This approach can be useful when the text needs to relate directly to specific data points although the method requires updating if the data limits change, making it less flexible for dynamic plots.

Method 4: Using Text Properties with Aspect Ratio

To ensure the text is aligned correctly with respect to the corner in an equal aspect figure, you might also consider setting the aspect manually and using the text() properties that manage alignment.

Here’s an example:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.set_aspect(1.0)

# Plot your data ...

# Place text at the corner
ax.text(0.95, 0.05, 'Data Point', transform=ax.transAxes, ha='right', va='bottom')

plt.show()

The result will be a plot with ‘Data Point’ annotated at the bottom right corner with respect to the axes box.

This method combines the control of aspect ratio with the ease of placing text using the axes transform, which is very useful for ensuring consistency in presentation style throughout different plots.

Bonus One-Liner Method 5: Using figtext()

The figtext() function allows adding text at an arbitrary location on the figure, making it ideal for a quick, one-liner addition of text to the corner.

Here’s an example:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.set_aspect('equal')

# Adding text at the bottom right corner in one line
plt.figtext(0.95, 0.05, 'Data Point', ha='right', va='bottom')

plt.show()

The output displays the text at the bottom right of the figure, regardless of the axes or data.

This method is very convenient for quick annotations when alignment and precision are not critical, offering a fast and easy way to annotate plots without much concern for figure resizing or reproportioning.

Summary/Discussion

  • Method 1: Using text() with Figure Coordinates. Strengths: Independent of axis scales and resilient to plot resizing. Weaknesses: Might require adjustments if figure size is drastically altered.
  • Method 2: Using annotate() with Axes Fraction. Strengths: Allows for offset adjustments and versatile positioning. Weaknesses: Layout changes can require repositioning of the text.
  • Method 3: Placing Text Relative to Data Coordinates. Strengths: Direct relation to data points. Weaknesses: Not practical for dynamically scaled plots as coordinates need to be recalculated.
  • Method 4: Using Text Properties with Aspect Ratio. Strengths: Maintains consistency across figures. Weaknesses: It requires manual aspect ratio setting which might not be ideal for some data sets.
  • Method 5: Using figtext(). Strengths: Simple, one-liner code for quick annotations. Weaknesses: Less precise placement and potential overlap with other figure elements if not carefully positioned.