Plotly：改变线交叉处的线颜色

Question

I have a dataframe with two data series and a date. I want to plot it and change the line colors when the data series are crossing.

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

data = {'Date': ['1.1.2020', '2.1.2020', '3.1.2020', '4.1.2020', '5.1.2020'],
    'S1': [20, 21, 19, 18, 20],
    'S2': [10, 11, 30, 18, 10]}
df = pd.DataFrame(data)

df['color']= df.apply(lambda row: 'black' if row['S1'] >= row['S2'] else 'red', axis=1)

fig = make_subplots(rows=1, cols=1, 
                vertical_spacing=0.05,
                shared_xaxes=True)

fig.add_trace(go.Scatter(
     x=df['Date'],
     y=df['S1'],
     line=dict(color='blue')),
     row=1, col=1)

fig.add_trace(go.Scatter(
     x=df['Date'],
     y=df['S2'],
     line=dict(color=list(df['color']))),
     row=1, col=1)

fig.show()

This is how it should look: Yet, the code does not work. Any hints?

Edit to increase solution space: It can also look like that2

@Lemon: if I run your proposed solution using that data set

data = {'Date': ['1.1.2020', '2.1.2020', '3.1.2020', '4.1.2020', '5.1.2020', '6.1.2020', '7.1.2020', '8.1.2020', '9.1.2020', '10.1.2020'],
'S1': [20, 21, 19, 18, 20, 19, 18, 20, 20, 20],
'S2': [10, 11, 30, 18, 10, 10, 17, 30, 20, 10]}

I will get the following result. I have not figured out a way to remove the red line between the crossings. Any hints?

Working Solution

based on lemons suggestion, here is the solution, that works for me

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

data = {'Date': ['1.1.2020', '2.1.2020', '3.1.2020', '4.1.2020', 
'5.1.2020', '6.1.2020', '7.1.2020', '8.1.2020', '9.1.2020', 
'10.1.2020'],
'S1': [20, 21, 19, 18, 20, 19, 18, 20, 20, 20],
'S2': [10, 11, 30, 18, 10, 10, 17, 30, 20, 10]}
df = pd.DataFrame(data)

df['crossing']= df.loc[(df.S1 < df.S2) | (df.S1.shift(1) < 
df.S2.shift(1)) | (df.S1.shift(-1) < df.S2.shift(-1)), 'S2']

fig = go.Figure()
fig.add_trace(go.Scatter(
  x=df['Date'],
  y=df['S1'],
  line=dict(color='blue')))

fig.add_trace(go.Scatter(
  x=df['Date'],
  y=df['S2'],
  line=dict(color='black')))

fig.add_trace(go.Scatter(
  x=df['Date'],
  y=df['crossing'],
  line=dict(color='red')))

fig.show()

Resulting chart

Answer 1

My initial idea followed the same r-beginners's idea, though what you can do to have a longer red line is detecting the rows when S2>S1 and a range of (-1, 1) centered on your " S2>S1 " rows, using:

df.loc[(df.S1 < df.S2) | 
       (df.S1.shift(1) < df.S2.shift(1)) | 
       (df.S1.shift(-1) < df.S2.shift(-1)), <the field you want to select>]

Final code should look like:

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

data = {'Date': ['1.1.2020', '2.1.2020', '3.1.2020', '4.1.2020', '5.1.2020'],
    'S1': [20, 21, 19, 18, 20],
    'S2': [10, 11, 30, 18, 10]}
df = pd.DataFrame(data)

df['color']= df.apply(lambda row: 'black' if row['S1'] >= row['S2'] else 'red', axis=1)

fig = make_subplots(rows=1, cols=1, 
                vertical_spacing=0.05,
                shared_xaxes=True)

fig.add_trace(go.Scatter(
     x=df['Date'],
     y=df['S1'],
     line=dict(color='blue')),
     row=1, col=1)

fig.add_trace(go.Scatter(
     x=df['Date'],
     y=df['S2'],
     line=dict(color='black')),
     row=1, col=1)

fig.add_trace(go.Scatter(
     x=df.loc[(df.S1 < df.S2) | (df.S1.shift(1) < df.S2.shift(1)) | (df.S1.shift(-1) < df.S2.shift(-1)), 'Date'],
     y=df.loc[(df.S1 < df.S2) | (df.S1.shift(1) < df.S2.shift(1)) | (df.S1.shift(-1) < df.S2.shift(-1)), 'S2'],
     line=dict(color='red')),
     row=1, col=1)

fig.show()