Here is my code:
\begin{figure*}
\begin{center}
\includegraphics[width=1in]{ch_f_25-eps-converted-to.pdf}\includegraphics[width=1in]{ch_f_30-eps-converted-to.pdf}\includegraphics[width=1in]{ch_f_35-eps-converted-to.pdf}
\includegraphics[width=1in]{ch_o_25-eps-converted-to.pdf}\includegraphics[width=1in]{ch_o_30-eps-converted-to.pdf}\includegraphics[width=1in]{ch_o_35-eps-converted-to.pdf}
\includegraphics[width=1in]{ch_s_25-eps-converted-to.pdf}\includegraphics[width=1in]{ch_s_30-eps-converted-to.pdf}\includegraphics[width=1in]{ch_s_35-eps-converted-to.pdf}
\includegraphics[width=1in]{c25-eps-converted-to.pdf}\includegraphics[width=1in]{c30-eps-converted-to.pdf}\includegraphics[width=1in]{c35-eps-converted-to.pdf}
\includegraphics[width=1in]{smf_ch25-eps-converted-to.pdf}\includegraphics[width=1in]{smf_ch30-eps-converted-to.pdf}\includegraphics[width=1in]{smf_ch35-eps-converted-to.pdf}
\includegraphics[width=1in]{gtr_ch_25.eps}\includegraphics[width=1in]{gtr_ch_30.eps}\includegraphics[width=1in]{gtr_ch_35.eps}\includegraphics[width=1in]{gtr_ch_40.eps}
\end{center}
\end{figure*}
So I have 6 rows and each row has three subfigures. So my question is how can I code in order to add the notes above each row and column to have something like this:
25 30 35
a x x x
b x x x
c x x x
d x x x
e x x x
f x x x
The above is not codes but the form that I want, where the "x" are subfigures. So I want to add the "20", "25", "30" and "a" to "f" as labels to make it like a table besides the subfigures.
How can I achieve this? Thanks in advance.

