A particular type of vertex.spells data leads to the "sliceRenderData.node[item[direction][0]] is undefined (line 1285)" error in the browser.
The data in vertex_spells_no_error.csv, which has onsets and termini for each individual year, works fine. The dataset in vertex_spells_error.csv has the min(onset) and max(terminus) for each of the vertex_ids in the first dataset. The aim is to reduce the number of spells and to provide a less "jumpy" visualization. I do not understand why dataset 1 works and dataset 2 does not.
The code for generating the network is here.