What does 'partial_cluster_label' vertex attribute refer to?

[vapniks]

In the mapper graph examples on this webpage: https://giotto-ai.github.io/gtda-docs/latest/notebooks/mapper_quickstart.html
when I hover my mouse pointer over the graph vertices, they almost all report 'Partial cluster label: 0' (I found one vertex with 'Partial cluster label: -1').
I made a mapper graph with some data I had, with 10989 rows and 200 columns, and in that case all the nodes report 'Partial cluster label: -1'
I thought each node should have a different partial cluster label, so why are they all the same? Is it a bug?