referencing a knitr figure in text from lyx

Question

I am using lyx to write an rnwfile. In my lyx text, I would like to reference a figure generated by a knitr code chunk that has label "chunk1".

If I write the rnw in Rstudio, I would reference the figure by \ref{fig:chunk1} in my text. However in lyx, the label "chunk1" or "fig:chunk1" does not appear in the cross-Reference window.

Can anyone help me with this? Thanks a lot!