>> daily_close_df.plot()

ax = daily_close_df[a_symbol.name + '_close'].plot(label=a_symbol.name)
daily_close_df[b_symbol.name + '_close'].plot(label=b_symbol.name, ax=ax, secondary_y=True)

ax.legend([ax.get_lines()[0], ax.right_ax.get_lines()[0]], [a_symbol.name, b_symbol.name])