pct parameter in transform_calculate

I'm trying to adapt the code from the following post: https://stackoverflow.com/questions/56358977/how-to-show-a-histogram-of-percentages-instead-of-counts-using-altair.

I looked through the documentation for the [transform_calculate](https://altair-viz.github.io/user_guide/generated/toplevel/altair.Chart.html#altair.Chart.transform_calculate) to find an explanation of the `pct` parameter but the search came up empty. Is it possible to update the documentation?

Alternatively, is there a different/better way to plot a histogram of percentages instead of counts using Altair?