The size of the plot area (in pixels) can be calculated by getting the value from the Axes object (ʻax) as follows: In addition, since Japanese is used in the figure, in the Google Colab. Environment etc., please execute ! Pip install japanize-matplotlib` first to install the library.

Calculate plot area size (in pixels)

import numpy as np
import japanize_matplotlib 
import matplotlib.pyplot as plt

x = np.linspace(0,20,100) #Sample data for drawing
y = x ** 0.5              #Sample data for drawing

fig = plt.figure(dpi=100, figsize=(4,3))
ax = plt.gca()
ax.plot(x,y)
ax.set_xlabel('input',fontsize=12)
ax.set_ylabel('output',fontsize=12)
plt.savefig('foo.png')

#Figure (whole image) size
fig_w_px = int(fig.get_figwidth()  * fig.get_dpi())  
fig_h_px = int(fig.get_figheight() * fig.get_dpi())
print(f'■ Fig size   {fig_w_px} x {fig_h_px} [px]')

#Axis (plot area) size
ax_size_inch = ax.figure.get_size_inches()
ax_w_inch = ax_size_inch[0] * (ax.figure.subplotpars.right - ax.figure.subplotpars.left)
ax_h_inch = ax_size_inch[1] * (ax.figure.subplotpars.top - ax.figure.subplotpars.bottom)
ax_w_px = int( ax_w_inch * fig.get_dpi() )  
ax_h_px = int( ax_h_inch * fig.get_dpi() )
print(f'■ Axis size  {ax_w_px } x {ax_h_px } [px]')

The execution result is as follows. Also, if you open the file output foo.png with image editing software etc. and check the size of the plot area, it surely matches the following result.

Execution result

■ Fig size   400 x 300 [px]
■ Axis size  310 x 226 [px]

Program description

--ʻAx.figure.get_size_inches () `, the size of the area including the overall size of the Axes object (= ** scales and labels for each axis (characters such as" input "and" output ") ** ) Is obtained in inches.

--In ʻax.figure.subplotpars.left, it is a value ($ 0.0 $ to $ 1.0 $) that represents the ** left edge position ** of the plot area when the entire width of the Axes object is ** $ 1.0 $. Also, ʻax.figure.subplotpars.right is a value that represents ** the rightmost position of the plot area **. --Here, the difference between the two ʻax.figure.subplotpars.right --ax.figure.subplotpars.left` is the width of the plot area where the ** width of the Axes object is ** $ 1.0 $ It represents the ratio of **.

--In order to obtain the width of the plot area in ** inches **, the total width of the Axes object ʻax_size_inch [0] (in inches) and the width (ratio) of the plot area ʻax.figure.subplotpars.right - Multiply by ax.figure.subplotpars.left and store in ʻax_w_inch`.

--In order to convert inches to pixels, ʻax_w_inch is multiplied by the resolution fig.get_dpi () and stored in ʻax_w_px.

I want to draw a graph by specifying the size of the plot area in pixels

I was able to ** find the size of the plot area in pixels **.

This time, on the contrary, ** specify the size of the plot area in pixels ** and draw a graph. The following code is a sample that draws a graph so that the size of the plot area is $ 400 \ times 300 $ (px).

--Reference: Demo Fixed Size Axes @ matplotlib.org

The cause is unknown, but at the end of the program, when the size of the plot area is recalculated and output, it becomes ʻAxis size 387 x 302 [px] `. However, when I measure the size of the plot area for the actually output image (foo.png), it is correctly $ 400 \ times 300 $ (px).

Graph output by specifying the size of the plot area in pixels

import numpy as np
import japanize_matplotlib 
import matplotlib.pyplot as plt
from mpl_toolkits.axes_grid1 import Divider, Size #add to
from mpl_toolkits.axes_grid1.mpl_axes import Axes #add to

x = np.linspace(0,20,100) #Sample data for drawing
y = x ** 0.5              #Sample data for drawing

ax_w_px = 400  #Specify the width of the plot area in pixels
ax_h_px = 300  #Specify the height of the plot area in pixels

#Processing for size specification ↓ ↓ From here ↓ ↓
fig_dpi = 100
ax_w_inch = ax_w_px / fig_dpi
ax_h_inch = ax_h_px / fig_dpi
ax_margin_inch = (0.5, 0.5, 0.5, 0.5)  # Left,Top,Right,Bottom [inch]

fig_w_inch = ax_w_inch + ax_margin_inch[0] + ax_margin_inch[2] 
fig_h_inch = ax_h_inch + ax_margin_inch[1] + ax_margin_inch[3]

fig = plt.figure( dpi=fig_dpi, figsize=(fig_w_inch, fig_h_inch))
ax_p_w = [Size.Fixed(ax_margin_inch[0]),Size.Fixed(ax_w_inch)]
ax_p_h = [Size.Fixed(ax_margin_inch[1]),Size.Fixed(ax_h_inch)]
divider = Divider(fig, (0.0, 0.0, 1.0, 1.0), ax_p_w, ax_p_h, aspect=False)
ax = Axes(fig, divider.get_position())
ax.set_axes_locator(divider.new_locator(nx=1,ny=1))
fig.add_axes(ax)
#Processing for size specification ↑↑ Up to here ↑↑

ax.plot(x,y)
ax.set_xlabel('input',fontsize=12)
ax.set_ylabel('output',fontsize=12)
plt.savefig('foo.png')

#Figure (whole image) size
fig_w_px = int(fig.get_figwidth()  * fig.get_dpi())  
fig_h_px = int(fig.get_figheight() * fig.get_dpi())
print(f'■ Fig size   {fig_w_px} x {fig_h_px} [px]')

#Axis (plot area) size
ax_size_inch = ax.get_figure().get_size_inches()
ax_w_inch = ax_size_inch[0] * (ax.figure.subplotpars.right - ax.figure.subplotpars.left)
ax_h_inch = ax_size_inch[1] * (ax.figure.subplotpars.top - ax.figure.subplotpars.bottom)
ax_w_px = int( ax_w_inch * fig.get_dpi() )  
ax_h_px = int( ax_h_inch * fig.get_dpi() )
print(f'■ Axis size  {ax_w_px } x {ax_h_px } [px]') 
#The print output above does not result in 400 x 300,
#Actual image ('foo.png'), The size of the plot area is 400 x 300.