How can I include values outside of the domain in a histogram?

[llimllib]

Let's say I have a dataframe with a series whose values are [1,1,2,2,3,4,4,4,4].

I'd like to create a histogram with three bins: 1, 2, and >= 3

I've been reading the bin and scale docs and I can't find any way to do this; can somebody help me figure it out?

My current try is:

df = pd.DataFrame(data={"a": [1,1,2,2,3,4,4,4,4]})
alt.Chart(df).mark_bar().encode(
    alt.X(
        "a:Q",
        bin=alt.Bin(extent=[1,3]),
        scale=alt.Scale(clamp=True),
    ),
    y="count()",
)

Also I'd like to label the last bin appropriately (>=3), but that only comes after I figure out how to make the last bin hold the data I want it to hold.

But clamping the scale doesn't work - I'm guessing it gets applied after the bin transformation?

Obviously I could do this in pandas myself before graphing, but I was hoping to accomplish it in altair.