Update 8_many_atoms.md

src/8_many_atoms.md

@@ -101,6 +101,9 @@ pyplot.vlines([-pi, pi], 0, 2.2, linestyles='dashed')
 pyplot.legend()
 pyplot.xlim(-1.75*pi, 3.5*pi)
 pyplot.ylim(bottom=0)
+ax.annotate(s='', xy=(-pi, -.15), xytext=(pi, -.15),
+            arrowprops=dict(arrowstyle='<->', shrinkA=0, shrinkB=0))
+ax.text(0, -.25, '1st Brillouin zone', ha='center')
 draw_classic_axes(ax, xlabeloffset=.2)
 ```