r - knitr: how to reference a plot from a multi-plot chunk -

i have .rnw document want reference plot multi-plot chunk. how do this?

example:

\documentclass{article} \begin{document}  <<single_chunk, fig.cap="hi">>= plot(1:5) @  can reference single chunk fine! see \ref{fig:single_chunk}.  <<multichunk, fig.cap="hello">>= plot(1:10) plot(10:1) @  first figure great, \ref{fig:multichunk}. try again \ref{fig:multichunk-1}.  \end{document} 

both these attempts result in ??.

just have @ generated *.tex file! here's relevant part (i took freedom align bit more nicely knit does):

\begin{knitrout} \definecolor{shadecolor}{rgb}{0.969, 0.969, 0.969}\color{fgcolor}\begin{kframe} \begin{alltt} \hlkwd{plot}\hlstd{(}\hlnum{1}\hlopt{:}\hlnum{10}\hlstd{)} \end{alltt} \end{kframe} \begin{figure}   \includegraphics[width=\maxwidth]{figure/multichunk-1}   \caption[hello]{hello}   \label{fig:multichunk1} \end{figure}  \begin{kframe}\begin{alltt} \hlkwd{plot}\hlstd{(}\hlnum{10}\hlopt{:}\hlnum{1}\hlstd{)} \end{alltt} \end{kframe} \begin{figure}   \includegraphics[width=\maxwidth]{figure/multichunk-2}   \caption[hello]{hello}   \label{fig:multichunk2} \end{figure} 

so, if closely, you'll note multiplot figures named fig:multichunk1 , fig:multichunk2. , indeed, if reference these (\ref{fig:multichunk1}, ...), works fine.