How to add a title to each subplot

ax.set_title() should set the titles for separate subplots:

import matplotlib.pyplot as plt

if __name__ == "__main__":
    data = [1, 2, 3, 4, 5]

    fig = plt.figure()
    fig.suptitle("Title for whole figure", fontsize=16)
    ax = plt.subplot("211")
    ax.set_title("Title for first plot")
    ax.plot(data)

    ax = plt.subplot("212")
    ax.set_title("Title for second plot")
    ax.plot(data)

    plt.show()

Can you check if this code works for you? Maybe something overwrites them later?

A shorthand answer assuming
import matplotlib.pyplot as plt:

plt.gca().set_title('title')

as in:

plt.subplot(221)
plt.gca().set_title('title')
plt.subplot(222)
etc...

Then there is no need for superfluous variables.

If you want to make it shorter, you could write :

import matplotlib.pyplot as plt
for i in range(4):
    plt.subplot(2,2,i+1).set_title(f'Subplot n°{i+1}')
plt.show()

It makes it maybe less clear but you don’t need more lines or variables

In case you have multiple images and you want to loop though them and show them 1 by 1 along with titles – this is what you can do. No need to explicitly define ax1, ax2, etc.

1. The catch is you can define dynamic axes(ax) as in Line 1 of code
   and you can set its title inside a loop.
2. The rows of 2D array is length (len) of axis(ax)
3. Each row has 2 items i.e. It is list within a list (Point No.2)
4. set_title can be used to set title, once the proper axes(ax) or subplot is selected.

import matplotlib.pyplot as plt    
fig, ax = plt.subplots(2, 2, figsize=(6, 8))  
for i in range(len(ax)): 
    for j in range(len(ax[i])):
        ## ax[i,j].imshow(test_images_gr[0].reshape(28,28))
        ax[i,j].set_title('Title-' + str(i) + str(j))

fig, (ax1, ax2, ax3, ax4) = plt.subplots(nrows=1, ncols=4,figsize=(11, 7))

grid = plt.GridSpec(2, 2, wspace=0.2, hspace=0.5)

ax1 = plt.subplot(grid[0, 0])
ax2 = plt.subplot(grid[0, 1:])
ax3 = plt.subplot(grid[1, :1])
ax4 = plt.subplot(grid[1, 1:])

ax1.title.set_text('First Plot')
ax2.title.set_text('Second Plot')
ax3.title.set_text('Third Plot')
ax4.title.set_text('Fourth Plot')

plt.show()

A solution I tend to use more and more is this one:

import matplotlib.pyplot as plt

fig, axs = plt.subplots(2, 2)  # 1
for i, ax in enumerate(axs.ravel()): # 2
    ax.set_title("Plot #{}".format(i)) # 3

1. Create your arbitrary number of axes
2. axs.ravel() converts your 2-dim object to a 1-dim vector in row-major style
3. assigns the title to the current axis-object

You are able to give every graph a different title and label by Iteration only.

titles = {221: 'First Plot', 222: 'Second Plot', 223: 'Third Plot', 224: 'Fourth Plot'}
fig = plt.figure()
for x in range(221,225):
  ax = fig.add_subplot(x)
  ax.title.set_text(titles.get(x))

plt.subplots_adjust(left=0.1,
                    bottom=0.1, 
                    right=0.9, 
                    top=0.9, 
                    wspace=0.4, 
                    hspace=0.4)
plt.show()

Output:

As of matplotlib 3.4.3, the Figure.add_subplot function supports kwargs with title as:

fig.add_subplot(311, title="first")
fig.add_subplot(312, title="second")

For completeness, the requested result can also be achieve without explicit reference to the figure axes as follows:

import matplotlib.pyplot as plt

plt.subplot(221)
plt.title("Title 1")

plt.subplot(222)
plt.title("Title 2")

plt.subplot(223)
plt.title("Title 3")

plt.subplot(224)
plt.title("Title 4")

Use plt.tight_layout() after the last plot if you have issues with overlapping labels.