Cell-level custom properties

If I understand correctly Google Visualizations supports custom properties at the cell level. However, this package seems to only provide APIs for setting them at the level of the entire column or the entire row.

Did I miss a way to cell-level properties or they are indeed not supported at the moment?

At the very least, this issue should be documented and if the capability to set cell-level custom properties is not available right now it should definitely be added...