chart.py

# https://altair-viz.github.io/gallery/multiline_tooltip.html?highlight=tooltip

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

np.random.seed(42)
source = pd.DataFrame(np.cumsum(np.random.randn(100, 3), 0).round(2),
                    columns=['A', 'B', 'C'], index=pd.RangeIndex(100, name='x'))
source = source.reset_index().melt('x', var_name='category', value_name='y')

# Create a selection that chooses the nearest point & selects based on x-value
nearest = alt.selection(type='single', nearest=True, on='mouseover',
                        fields=['x'], empty='none')

def make_chart(source)

    # The basic line
    line = alt.Chart(source).mark_line(interpolate='basis').encode(
        x='x:Q',
        y='y:Q',
        color='category:N'
    )

    # Transparent selectors across the chart. This is what tells us
    # the x-value of the cursor
    selectors = alt.Chart(source).mark_point().encode(
        x='x:Q',
        opacity=alt.value(0),
    ).add_selection(
        nearest
    )

    # Draw points on the line, and highlight based on selection
    points = line.mark_point().encode(
        opacity=alt.condition(nearest, alt.value(1), alt.value(0))
    )

    # Draw text labels near the points, and highlight based on selection
    text = line.mark_text(align='left', dx=5, dy=-5).encode(
        text=alt.condition(nearest, 'y:Q', alt.value(' '))
    )

    # Draw a rule at the location of the selection
    rules = alt.Chart(source).mark_rule(color='gray').encode(
        x='x:Q',
    ).transform_filter(
        nearest
    )

    # Put the five layers into a chart and bind the data
    alt.layer(
        line, selectors, points, rules, text
    ).properties(
        width=600, height=300)

    return alt