在 Altair 中更改 X 轴

Question

我想用选择器填充图表，如下例所示。关于如何让它在多面图表中工作的任何提示？

np.random.seed(42)
source = pd.DataFrame(np.cumsum(np.random.rand(8, 4), 0).round(2),
                    columns=['A', 'B', 'C', 'D'], index=pd.RangeIndex(8, name='x'))
source = source.reset_index().melt('x', var_name='category', value_name='y')
    
xRange= pd.DataFrame(np.linspace(min(source['x']), max(source['x']), num=100), columns=['x'])

pts = alt.selection_multi(fields=['x'], nearest=True, on='click',empty='none')
    
# The basic line
main = alt.Chart(source).mark_line(interpolate='basis').encode(
    x='x:Q',
    y='y:Q',
).transform_filter(
   alt.FieldEqualPredicate(field='category', equal='A')
)

line = alt.Chart(source).mark_line(color='Maroon').encode(
    x='x:Q',
    y='y:Q',
).transform_filter(
   alt.FieldEqualPredicate(field='category', equal='B')
)

# Transparent selectors across the chart. This is what tells us
# the x-value of the cursor
selectors = alt.Chart(xRange).mark_rule(size=2).encode(
    x='x:Q',
    #y='y:Q',
    #opacity=alt.value(0.4),
    opacity = alt.condition(pts, alt.value(1.0), alt.value(0.2))
).add_selection(pts)

position = alt.Chart(xRange).mark_text(
    align='right', dy=140, dx=-8, fontSize=14).encode(
    x=alt.X('x'),
    text=alt.Text('x',format='.1f')
).transform_filter(pts)
        
alt.vconcat(
    main + selectors + position,
    line + selectors + position
)

但理想情况下使用 facet，但我还没有找到一种方法，您只能使用单个 DataFrame/源。有没有办法使用 alt.sequence 的 impute 在 x 轴上生成额外的点？

pts = alt.selection_multi(fields=['x'], nearest=True, on='click',empty='none')

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

# Transparent rules across the chart. 
rules = alt.Chart().mark_rule(size=2).encode(
    x='x:Q',
    opacity = alt.condition(pts, alt.value(1.0), alt.value(0.3))
).add_selection(pts)

text = alt.Chart().mark_text(
    align='right', dy=140, dx=-8, fontSize=14).encode(
    x=alt.X('x'),
    text=alt.Text('x',format='.1f')
).transform_filter(pts)
        
alt.layer(line, rules, text, data=source).facet(
    'category:N',
    columns=2
)

Answer 1

您可以使用序列生成器。它与您已经拥有的几乎相同：

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

np.random.seed(42)
source = pd.DataFrame(np.cumsum(np.random.rand(8, 4), 0).round(2),
                    columns=['A', 'B', 'C', 'D'], index=pd.RangeIndex(8, name='x'))
source = source.reset_index().melt('x', var_name='category', value_name='y')
    
# xRange= pd.DataFrame(np.linspace(min(source['x']), max(source['x']), num=100), columns=['x'])
xRange = alt.sequence(0, 7.1, 0.1, as_='x')

pts = alt.selection_multi(fields=['x'], nearest=True, on='mouseover',empty='none')

# The basic line
line = alt.Chart().mark_line(interpolate='linear').encode(
    x='x:Q',
    y='y:Q',
)

# Transparent rules across the chart. 
rules = alt.Chart(xRange).mark_rule(size=2).encode(
    x='x:Q',
    opacity = alt.condition(pts, alt.value(1.0), alt.value(0.3))
).add_selection(pts)

text = alt.Chart(xRange).mark_text(
    align='right', dy=140, dx=-8, fontSize=14).encode(
    x=alt.X('x:Q'),
    text=alt.Text('x:Q',format='.1f')
).transform_filter(pts)
        
alt.layer(line, rules, text, data=source).facet(
    'category:N',
    columns=2
)