How do you format the tooltip on the sum column in a waterfall chart?

[T.H.]

On a waterfall chart we can control the formatting of tooltips. For example, an item is configured like this (in TypeScript):

{
  label: label,
   value: value,
   toolText: `${fmt.format(value)}`
 }

However, this does not work for a sum column. How can I achieve the above (custom formatting) of a value in the tooltip on a sum column?