Altair-viz 1: Introduction and base charts

But, tell me what is Altair? 🤔

Altair is a python library for data viz, built upon Vega and Vega-lite

Why should you care

• You need to create good looking or complex charts without too much boilerplate
• You don’t need to draw graphs
• You want easy to setup and advanced interaction
• Great integration in streamlit and jupyter

I have seaborn, why should I choose Altair?

• Altair is slightly more beautiful for the same amount of code (IMHO)
• Layout management is easier in Altair
• You need to transform or filter data for the chart
• You want conditional rendering (color extreme values, etc)

I am not convinced, show me what you got

OK fine, I know that you are a busy person so if I do not show you some beautiful stuff right now, you might leave before the end of the article.

Here we are:

Base charts

Preliminaries

Start by importing pandas and altair, build a dummy dataframe and instantiate a Chart object.

import pandas as pd
import altair as alt

df = pd.DataFrame({
    "x": [1, 2, 3, 4, 5, 6], 
    "y": [15, 16, 5, 7, 3, 30],
    "z": [0.1, 0.4, 0.6, 0.2, 0.15, 0.8], 
    "names": ["Pierre", "Paul", "Jacques", "Pierre", "Paul", "Jacques"]
})

chart = alt.Chart(df).encode(
    x="x:Q", 
    y="y:Q"
)

alt.Chart(data) creates a Chart instance, which will use columns from df as core data. By assigning encodings to the chart, one decides how data is displayed: which goes on the X axis, which goes on Y. There is several more encodings: color, theta, tooltip, etc. Moreover, what is the weird letter Q at the end of x:Q and y:Q? We’ll cover that later.

Now, you should be willing to display something: dots, bars, area, grid?

For that, once you defined proper encodings, you can tell altair what to draw:

chart.mark_point()

chart.mark_bar()

chart.mark_line()

Recap

• Altair is a framework of data visualization in python.
• It uses pandas DataFrame as data sources, and displaying charts are done in a triple step fashion:
  • Create the chart object chart = alt.Chart(df)
  • Tell altair what columns should be used, and how by specifying encodings: chart_encoded = chart.encode(x=<column A>, y=<column B>)
  • Tell altair what to draw by specifying a mark: chart_encoded.mark_*()

• data-visualization
• altair-viz
• python