Vega-Lite - One plot for multiple datasets

Question

I'm working on a package for Julia with the goal of doing quick plots using Vega-Lite as backend. As people familiar with Matplotlib know, it is very common to have different sets for vectors, and plot all of them in the same figure, each with it's own label. For example:

x = range(0,10)
y = np.random.rand(10)
w = range(0,5)
z = np.random.rand(5)
plt.plot(x,y,label = 'y')
plt.plot(w,z,label = 'z')
plt.legend()

What I'd like to know is how can I do something similar, but using Vega-Lite (or Altair). I know that I can do two separate plots and then add one over another. My problem is mainly about how to get the legends to work, since to get a legend, one usually needs another field such as "color", pointing to another field in the dataframe.

I've seen similar posts, but dealing with the question of posting data from different columns. The answer to this case is basically to use the Fold Transform. But in my question this doesn't quite work, because I'm more interested in starting from two different plots, possibly using two different datasets, so "merging" the datasets is not a good solution.

Answer 1

You can take advantage of the fact that in composite charts, Vega-Lite uses shared scales by default. If you assign the color, shape, strokeDash, etc. to a unique value for each layer, an appropriate legend will be generated automatically.

Here is an example, using Altair to generate the Vega-Lite specification:

import pandas as pd
import numpy as np
import altair as alt

x = np.linspace(0, 10)

df1 = pd.DataFrame({
    'x': x,
    'y': np.sin(x)
})
df2 = pd.DataFrame({
    'x': x,
    'y': np.cos(x)
})

chart1 = alt.Chart(df1).transform_calculate(
    label='"sine"'
).mark_line().encode(
    x='x',
    y='y',
    color='label:N'
)

chart2 = alt.Chart(df2).transform_calculate(
    label='"cosine"'
).mark_line().encode(
    x='x',
    y='y',
    color='label:N'
)

alt.layer(chart1, chart2)