How to put a bounding box around groups of subplots while using gridspec?

Question

My goal is to put a bounding box around each set of 4 subplots. I can't seem to figure out how to do it with my gridspec subplots. The examples I have came across refer to individual subplots but not to the total gridspec.

(I chose to make the plots like this using gridspec so I could have control over the spacing between the groups of subplots)

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



fig = plt.figure(figsize=(16,16))

fig.suptitle(' title ', fontsize=12,
             bbox={'facecolor':'none', 'alpha':0.5, 'pad':5})

gs = gridspec.GridSpec(2, 2)


gs.update(top=.48,left=0.1, right=0.48, wspace=0.15,hspace=0.2)
ax1 = plt.subplot(gs[0])
ax1.set_title('axes title')

ax2 = plt.subplot(gs[1])
ax2.set_title('axes title')

ax3 = plt.subplot(gs[2])
ax3.set_title('axes title')

ax4 = plt.subplot(gs[3])
ax4.set_title('axes title')

#New Gridspec
gs1 = gridspec.GridSpec(2, 2)
gs1.update(bottom=.53,left=0.55, right=0.9, hspace=0.2, wspace=.15)

ax5 = plt.subplot(gs1[0])
ax5.set_title('axes title')

ax6 = plt.subplot(gs1[1])
ax6.set_title('axes title')

ax7 = plt.subplot(gs1[2])
ax7.set_title('axes title')

ax8 = plt.subplot(gs1[3])
ax8.set_title('axes title')


#New Gridspec
gs2 = gridspec.GridSpec(2, 2)
gs2.update(top=.48,left=0.55, right=0.9, hspace=0.2, wspace=.15)

ax9 = plt.subplot(gs2[0])
ax9.set_title('axes title')

ax10 = plt.subplot(gs2[1])
ax8.set_title('axes title')

ax11 = plt.subplot(gs2[2])
ax11.set_title('axes title')

ax12 = plt.subplot(gs2[3])
ax12.set_title('axes title')

#New Gridspec

gs3 = gridspec.GridSpec(2, 2)
gs3.update(bottom=.53,left=0.1, right=0.48, wspace=0.15,hspace=0.2)

ax13 = plt.subplot(gs3[0])
ax13.set_title('axes title')

ax14 = plt.subplot(gs3[1])
ax14.set_title('axes title')

ax15 = plt.subplot(gs3[2])
ax15.set_title('axes title')

ax16 = plt.subplot(gs3[3])
ax16.set_title('axes title')



plt.savefig('test.png',bbox_inches='tight')

Answer 1

An option to create a frame or border around several subplots may be to add another axes to the figure which is a bit bigger than the extent of the respective gridspec region.

outergs = gridspec.GridSpec(1, 1)
outergs.update(bottom=.50,left=0.07, right=0.50,top=0.93)
outerax = fig.add_subplot(outergs[0])
outerax.tick_params(axis='both',which='both',bottom=0,left=0,
                    labelbottom=0, labelleft=0)

In order for this to work, all other axes must be created via
ax_i = fig.add_subplot(gs...) instead of ax_i = plt.subplot(gs...)

You may then of course modify the outeraxto your liking, e.g. changing its color via

outerax.set_facecolor(colors[i])
outerax.patch.set_alpha(0.3)

Complete code to reproduce the above:

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

fig = plt.figure(figsize=(9,9))
fig.suptitle(' title ', fontsize=12,
             bbox={'facecolor':'none', 'alpha':0.5, 'pad':5})

colors=["crimson", "indigo", "limegreen", "gold"]

for i in range(4):
    #outer
    outergs = gridspec.GridSpec(1, 1)
    outergs.update(bottom=(i//2)*.47+0.01,left=(i%2)*.5+0.02, 
                   top=(1+i//2)*.47-0.01,  right=(1+i%2)*.5-0.02)
    outerax = fig.add_subplot(outergs[0])
    outerax.tick_params(axis='both',which='both',bottom=0,left=0,
                        labelbottom=0, labelleft=0)
    outerax.set_facecolor(colors[i])
    outerax.patch.set_alpha(0.3)

    #inner
    gs = gridspec.GridSpec(2, 2)
    gs.update(bottom=(i//2)*.47+0.05,left=(i%2)*.5+0.08, 
                   top=(1+i//2)*.47-0.05,  right=(1+i%2)*.5-0.05,
                   wspace=0.35, hspace=0.35)
    for k in range(4):
        ax = fig.add_subplot(gs[k])
        ax.set_title('Axes Title {}'.format(k+1), color=colors[i])

plt.show()