Matplotlib - adding subplots to a subplot?

You can nest your GridSpec using SubplotSpec. The outer grid will be a 2 x 2 and the inner grids will be 2 x 1. The following code should give you the basic idea.

import matplotlib.pyplot as plt
import matplotlib.gridspec as gridspec

fig = plt.figure(figsize=(10, 8))
outer = gridspec.GridSpec(2, 2, wspace=0.2, hspace=0.2)

for i in range(4):
    inner = gridspec.GridSpecFromSubplotSpec(2, 1,
                    subplot_spec=outer[i], wspace=0.1, hspace=0.1)

    for j in range(2):
        ax = plt.Subplot(fig, inner[j])
        t = ax.text(0.5,0.5, 'outer=%d, inner=%d' % (i, j))
        t.set_ha('center')
        ax.set_xticks([])
        ax.set_yticks([])
        fig.add_subplot(ax)

fig.show()

You can now nest subplots with figure.subfigure: https://matplotlib.org/stable/gallery/subplots_axes_and_figures/subfigures.html

You may also find subplot_mosaic useful, as it supports nesting: https://matplotlib.org/stable/tutorials/provisional/mosaic.html

While nesting is always a bit of a pain, arguably both of these methods make the process easier and more flexible.

Taken from the examples above:

import matplotlib.pyplot as plt

fig = plt.figure(constrained_layout=True)

subfigs = fig.subfigures(2, 2)

for outerind, subfig in enumerate(subfigs.flat):
    subfig.suptitle(f'Subfig {outerind}')
    axs = subfig.subplots(2, 1)
    for innerind, ax in enumerate(axs.flat):
        ax.set_title(f'outer={outerind}, inner={innerind}', fontsize='small')
        ax.set_xticks([])
        ax.set_yticks([])
 
plt.show()

As I had to rely on Suever's answer but had to modify it to my needs, I thought I might contribute in case someone finds it helpful:

import matplotlib.pyplot as plt
import matplotlib.gridspec as gridspec

channelArrangement = [16, 17, 18 , 19 , 22, 25, 28 , 29 , 31]

fig = plt.figure(figsize=(10, 8))
outer = gridspec.GridSpec(1, 2, wspace=0.2, hspace=0.2)

for i in range(2):
    inner = gridspec.GridSpecFromSubplotSpec(3, 3, subplot_spec=outer[i],
                                             wspace=0.1, hspace=0.1)
    row     = 0
    col     = 0
    maxCol  = 3

    for chan in channelArrangement:
        ax = plt.Subplot(fig, inner[row,col])
        t= ax.text(0.5,0.5, 'outer=%d\nrow=%d\ncol=%d' % (i,row,col))
        ax.set_xticks([])
        ax.set_yticks([])
        t.set_ha('center')
        fig.add_subplot(ax)
        col += 1
        if col == maxCol:
            col = 0
            row += 1
plt.show()