The Sleepwalk app, being used to explore the t-SNE rendition of the cord-blood data set from Figure 1. The plots here are snapshots of a running Sleepwalk app; a live version can be found at https://anders-biostat.github.io/sleepwalk/supplementary/. The red arrow shows the current mouse position. (A) By moving the mouse cursor through the embedding, we find, for example, that the CD4⁺ T cell cluster is very tight and homogeneous, as can be seen from the fact that all cells show a color indicating that they are all close to each other. (B) The monocyte cluster, in contrast, shows much more heterogeneity, when comparing the coloring at the same color scale: Now only a few monocytes are colored green and are hence as similar to the cell under the mouse cursor as most of the T cells were in A. (C) Placing the mouse on this small tip of the monocyte cluster reveals that the cells there are more similar to the T cells than to the other monocytes, indicating that the cluster boundary might be inaccurate in both the t-SNE rendition and the SNN clustering (Waltman and van Eck 2013) on which the Seurat workflow's cell-type assignment is based. (D) With the color scale set to a wider distance range, we can assess similarities between clusters: As expected, B cells are somewhat similar to T cells, less so to NK cells and monocytes, and distant to erythrocytes and the spiked-in mouse cells.