Description

When using make_subplots() with shared_xaxes=True, spike lines only appear on the subplot where the cursor is hovering, rather than showing across all subplots simultaneously.

Expected Behavior

When hovering over any subplot, a vertical spike line should appear at the same x-position across all subplots, similar to the behavior in TradingView or other financial charting tools.

Current Behavior

The spike line only appears on the subplot currently under the cursor. When moving to a different subplot, the spike line moves with the cursor instead of showing on all subplots.

Code Example

import plotly.graph_objects as go
from plotly.subplots import make_subplots
import pandas as pd

# Create sample data
df = pd.DataFrame({
    'x': pd.date_range('2024-01-01', periods=100),
    'y1': range(100),
    'y2': [x**2 for x in range(100)]
})

# Create subplots
fig = make_subplots(
    rows=2, cols=1,
    shared_xaxes=True,
    vertical_spacing=0.02
)

fig.add_trace(go.Scatter(x=df['x'], y=df['y1'], name='Line 1'), row=1, col=1)
fig.add_trace(go.Scatter(x=df['x'], y=df['y2'], name='Line 2'), row=2, col=1)

fig.update_xaxes(
    showspikes=True,
    spikemode='across',
    spikesnap='cursor',
    spikecolor='black',
    spikethickness=1
)

fig.update_layout(hovermode='x unified')
fig.show()

What I've Tried

1. hovermode='x unified' - Only unifies hover labels, not spike lines
2. fig.update_traces(xaxis='x') - Shows spikes across all subplots but breaks subplot layout (all data gets squeezed into first subplot's x-axis range)
3. Various combinations of spikemode, spikesnap, and matches='x' - No effect

Use Case

This feature is critical for financial/trading dashboards where users need to compare multiple indicators (price, volume, RSI, MACD, etc.) at the same point in time across multiple subplots.