Matplotlib Subplots with plt.subplots

Subplots with plt.subplots

• plt.subplots is used to draw subplots in a uniform state
• Returns a tuple of the canvas and a list of subplots
• The first number is the number of rows, and the second is the number of columns
• Default values are 1 when no parameters are passed
• figsize parameter specifies the size of the entire canvas
• sharex and sharey indicate whether to share the horizontal and vertical axis scales
• tight_layout function adjusts relative subplot sizes to prevent character overlap

Example Implementation

• The code generates a 2x5 grid of subplots, sharing x and y axes
• Titles and axis labels are added to each subplot
• Random scatter plots are drawn in each subplot

fig, axs = plt.subplots(2, 5, figsize=(10, 4), sharex=True, sharey=True)
fig.suptitle('Sample 1', size=20)
for i in range(2):
    for j in range(5):
        axs[i][j].scatter(np.random.randn(10), np.random.randn(10))
        axs[i][j].set_title('Row %d, Column %d' % (i+1, j+1))
        axs[i][j].set_xlim(-5, 5)
        axs[i][j].set_ylim(-5, 5)
        if i == 1:
            axs[i][j].set_xlabel('Horizontal')
        if j == 0:
            axs[i][j].set_ylabel('Vertical')
fig.tight_layout()

OO vs Pyplot modes

• subplots uses an object-oriented approach by creating one or more axes objects
• Then it performs the drawing operation on the corresponding subplot object
• subplot uses a pyplot-based approach by creating a new subplot in the specified location each time
• Subsequent drawing operations are then directed to the current subplot
• subplot is essentially a wrapper for Figure.add_subplot

Calling subplot

• When calling subplot, three digits are generally passed, representing total rows, total columns, and the index of the current subplot

plt.figure()
## Subplot 1
plt.subplot(2, 2, 1)
plt.plot([1, 2], 'r')
## Subplot 2
plt.subplot(2, 2, 2)
plt.plot([1, 2], 'b')
## Subplot 3
plt.subplot(224)  # When all three digits are less than 10, the middle comma can be omitted
plt.plot([1, 2], 'g')

Polar Coordinate System

• Polar coordinate plots can be created using the projection method

N = 150
r = 2 * np.random.rand(N)
theta = 2 * np.pi * np.random.rand(N)
area = 200 * r**2
colors = theta
plt.subplot(projection='polar')
plt.scatter(theta, r, c=colors, s=area, cmap='hsv', alpha=0.75)

Non-Uniform Subplots with GridSpec

• Non-uniformity has two meanings:
  • Different aspect ratios without spanning rows or columns
  • Subplots spanning rows or columns

add_gridspec

• add_gridspec can be used to specify the relative width (width_ratios) and relative height (height_ratios) parameters

fig = plt.figure(figsize=(10, 4))
spec = fig.add_gridspec(nrows=2, ncols=5, width_ratios=[1, 2, 3, 4, 5],
                          height_ratios=[1, 3])
fig.suptitle('Sample 2', size=20)
for i in range(2):
    for j in range(5):
        ax = fig.add_subplot(spec[i, j])
        ax.scatter(np.random.randn(10), np.random.randn(10))
        ax.set_title('Row %d, Column %d' % (i+1, j+1))
        if i == 1:
            ax.set_xlabel('Horizontal')
        if j == 0:
            ax.set_ylabel('Vertical')
fig.tight_layout()

Subplot Merging Through Slicing

• Subplot merging is possible using slicing to achieve the function of spanning graphs

fig = plt.figure(figsize=(10, 4))
spec = fig.add_gridspec(nrows=2, ncols=6, width_ratios=[2, 2.5, 3, 1, 1.5, 2], height_ratios=[1, 2])
fig.suptitle('Sample 3', size=20)

## Subplot 1
ax = fig.add_subplot(spec[0, :3])
ax.scatter(np.random.randn(10), np.random.randn(10))

## Subplot 2
ax = fig.add_subplot(spec[0, 3:5])
ax.scatter(np.random.randn(10), np.random.randn(10))

## Subplot 3
ax = fig.add_subplot(spec[:, 5])
ax.scatter(np.random.randn(10), np.random.randn(10))

## Subplot 4
ax = fig.add_subplot(spec[1, 0])
ax.scatter(np.random.randn(10), np.random.randn(10))

## Subplot 5
ax = fig.add_subplot(spec[1, 1:5])
ax.scatter(np.random.randn(10), np.random.randn(10))

fig.tight_layout()

Subplot Methods

• Commonly used methods on subplots including:
  • axhline: Draws horizontal lines on the subplot
  • axvline: Draws vertical lines on the subplot
  • axline: Draws lines in any direction on the subplot

fig, ax = plt.subplots(figsize=(4, 3))
ax.axhline(0.5, 0.2, 0.8)
ax.axvline(0.5, 0.2, 0.8)
ax.axline([0.3, 0.3], [0.7, 0.7])

Grid and Scale

• grid(True) adds a gray grid to the subplot

fig, ax = plt.subplots(figsize=(4, 3))
ax.grid(True)

• set_xscale sets the coordinate axis scale (e.g., logarithmic coordinates)

fig, axs = plt.subplots(1, 2, figsize=(10, 4))
for j in range(2):
    axs[j].plot(list('abcd'), [10**i for i in range(4)])
    if j == 0:
        pass
    else:
        axs[j].set_yscale('log')
fig.tight_layout()