A collapsed spectrum of the cube displayed in the upper-left viewer automatically appears in the 1D spectrum viewer, using the Maximum collapse method. The collapse method can be changed in the Viewer tab of the icon in the spectrum viewer. Additional spectra can be loaded into the spectrum viewer, as detailed in the linked documentation below.
Displaying Spectra (Specviz) <specviz-displaying>Documentation on displaying spectra in a 1D spectrum viewer.
There is one important difference when using the API to access Specviz from within Cubeviz. The functionality of the jdaviz.configs.specviz.helper.Specviz API can be accessed in Cubeviz via the ~jdaviz.configs.cubeviz.helper.Cubeviz.specviz attribute, e.g. cubeviz.specviz.get_spectra().