[Python] Display the Altair legend in the plot area

How To Place Legend Inside the Plot area in Altair

About this article

Recently, I like Altair, which is compatible with pandas/dataframe. However, the position of the legend is displayed in the plot with matplotlib, but it is outside with Altair, so I did not know how to adjust the position. I couldn't find the material in English, but I solved it by looking at the Vega-Lite document parsed by Altair, so I will write it as an article.

I would like to explain with a graph of Iris data from this site, which was very helpful. [Recommendation of Altair! Data visualization with Python] (https://qiita.com/dr666m1/items/b71bef31ba3a21868095)

BEFORE By default, it will appear on the right side スクリーンショット 2021-01-19 100349.png

import altair as alt
from vega_datasets import data
iris = data.iris()

alt.Chart(iris).mark_point().encode(
    alt.X("sepalLength", scale=alt.Scale(zero=False)),
    alt.Y("sepalWidth", scale=alt.Scale(zero=False)),
    color="species"
).interactive()

AFTER can be moved to any location in the plot area スクリーンショット 2021-01-19 101032.png


import altair as alt
from vega_datasets import data
iris = data.iris()

alt.Chart(iris).mark_point().encode(
    alt.X("sepalLength", scale=alt.Scale(zero=False)),
    alt.Y("sepalWidth", scale=alt.Scale(zero=False)),
    color=alt.Color("species", 
        legend=alt.Legend(orient='none', legendX=250, legendY=5,  ##If orient is set to none, legendX,legendY can be specified
                strokeColor='blue', fillColor='white', padding=10))　##Legend square setting
)

Legend setting points

As you can see from the code, you can make various fine adjustments with alt.Legend. You can adjust the position with LegendX and LegendY, but the point is to set oriend ='none' at that time. In addition, I do not know what the numerical values of x and y are ... I can do it somehow, but if anyone knows, please let me know.

Below, I will take a screenshot of the important parts of the Veg-Lite document that I referred to. Legend | Vega-Lite https://vega.github.io/vega-lite/docs/legend.html

スクリーンショット 2021-01-19 103539.png スクリーンショット 2021-01-19 103610.png スクリーンショット 2021-01-19 103720.png

Although it has nothing to do with this article, the examples of this Chinese site were substantial. Python Altair COVID-19 Epidemic Number Visualization Practice Economic Experience https://www.cnblogs.com/dotlink/archive/2004/01/13/covid-19.html