
\coordinate (AA) at ($(A1)-(\dx,0)$);
\node at ($(AA)-(1.2,0)$) {\small{$\mathbf{r_1}$}};
\node at ($(S14)+(-0.2,-0.2)$) {\small{$\mathbf{r_2}$}};
\node at ($(S24)+(0.2,-0.2)$) {\small{$\mathbf{r_m}$}};