[Therion] Centre lines and therion

Hello We (Wookey and I) are working on the Mulu data set which is using survex 3d files to import the survey legs. This shows the survey legs within the passage where it exists, but not when there is no pasage. However what we would really like to do is show the centreline when there is no passage, and not show it when there is passage drawn. Is there any way to do this thanks Andrew