Altair: How do I draw a multiline graph from dataframe columns?

[tweakimp]

I have a dataframe with multiple columns similar to this one:

import pandas as pd
import altair as alt
df = pd.DataFrame([[x ** 2, x ** 3] for x in range(10)], columns=["squared", "cubed"])
df.index.name = "x"

# print(df)
   squared  cubed
x
0        0      0
1        1      1
2        4      8
3        9     27
4       16     64
5       25    125
6       36    216
7       49    343
8       64    512
9       81    729

How do I plot these columns in the same graph? So far I can only do this:

chart = alt.Chart(df.reset_index()).mark_line().encode(x="x", y="squared").interactive()

How can I add the cubed column to this? What about even more columns?
What I see in the multiline examples are other ways of storing the data by having two columns with one having all values and one telling where they come from.
Also, is there no other way to use the index of the dataframe as the x-axis other than to reset the index and then use it as a normal column?

[jakevdp]

One way to do this is using a layered chart:

base = alt.Chart(df.reset_index()).encode(x='x')

alt.layer(
    base.mark_line(color='blue').encode(y='squared'),
    base.mark_line(color='red').encode(y='cubed')
)

That requires manual specification of colors.

A better way to do this is to reshape the data using pd.melt() to combine the multiple columns into one:

data = df.reset_index().melt('x')
alt.Chart(data).mark_line().encode(
    x='x',
    y='value',
    color='variable'
)

See Long-form vs Wide-form data for more details on the data formats that are best supported by Altair.

[tweakimp]

Thank you for your answer and keep up the good work :)