Altering the X-axis in Altair - facet

Question

I'd like to fill the charts with selectors like the example below. Any tips on how to get this to work in a faceted chart?
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
)
But ideally using facet, however i have not found a way around that you can only use a single DataFrame/source. Is there a way to use alt.sequence of impute to generate additional points on the x-axis?
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

You can use the sequence generator. It is almost the same to what you had already:
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
)