[Python] How can I keep distincts data points with similar X value?

I have a dataframe with 4 rows, two of which having the same value for the "a" field. I'd like to have a bar chart with 4 bars, one for each row.

Trying with Altair

source = pd.DataFrame({
'a': ['A', 'B', 'C', 'B'],
'b': [10, 20, 30, 15]
})

alt.Chart(source).mark_bar().encode(
x='a',
y='b'
)


This will result in a 3-bar chart, the values for 'B' having been added (20+15=35)

3-bar chart with Altair

What I was expecting is a 4-bar chart, like I get with:

source.plot(kind="bar", x="a", y="b")


4-bar chart plotting directly from Pandas

If each value must be unique, I guess I could use the index of the dataframe, but I have not found any way to get the labels from the 'a' column.

What would be the proper way to solve this?

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