How to shade a chart above a specific Y value in Python Plotly?

Plotly is a powerful plotting library in Python that enables you to create interactive visualizations. Sometimes you need to highlight specific regions of your chart by shading areas above or below certain Y values to emphasize important thresholds or ranges.

In this tutorial, we will show how to shade a chart above a specific Y value using Plotly's add_hrect() method.

Understanding the Methods

To shade chart areas, we use these key Plotly methods:

• add_hrect() ? Adds a horizontal rectangle to shade regions based on Y-axis values

• add_vline() ? Adds vertical reference lines

• plotly.express ? Creates the base scatter plot

Basic Example

Let's create a scatter plot and shade the area above a specific Y value ?

import plotly.express as px
import pandas as pd

# Create sample data
data = {
   'model': ['moto', 'sony', 'samsung', 'redmi'],
   'price': [20000, 25000, 30000, 28000],
   'year': [2018, 2017, 2015, 2019]
}

df = pd.DataFrame(data)

# Create scatter plot
fig = px.scatter(df, x='price', y='year', title='Phone Models by Price and Year')

# Add horizontal shading above year 2017
fig.add_hrect(
    y0=2017,  # Start Y value
    y1=2020,  # End Y value (above the data range)
    fillcolor="lightblue",
    opacity=0.3,
    line_width=0
)

# Optional: Add a reference line at the threshold
fig.add_hline(
    y=2017, 
    line_dash="dash", 
    line_color="blue",
    annotation_text="Threshold: 2017"
)

fig.show()

Advanced Shading with Multiple Regions

You can add multiple shaded regions with different colors and opacities ?

import plotly.express as px
import pandas as pd

# Create more sample data
data = {
    'month': ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun'],
    'sales': [100, 150, 120, 180, 200, 160],
    'target': [130, 130, 130, 130, 130, 130]
}

df = pd.DataFrame(data)

# Create line plot
fig = px.line(df, x='month', y='sales', title='Monthly Sales vs Target')
fig.add_scatter(x=df['month'], y=df['target'], mode='lines', name='Target', line_dash='dash')

# Shade area above target (good performance)
fig.add_hrect(
    y0=130,  # Target line
    y1=250,  # Upper limit
    fillcolor="green",
    opacity=0.2,
    line_width=0,
    annotation_text="Above Target",
    annotation_position="top right"
)

# Shade area below target (poor performance) 
fig.add_hrect(
    y0=0,    # Lower limit
    y1=130,  # Target line
    fillcolor="red",
    opacity=0.2,
    line_width=0,
    annotation_text="Below Target",
    annotation_position="bottom left"
)

fig.show()

Key Parameters

The add_hrect() method accepts these important parameters:

Conclusion

Use add_hrect() to shade chart regions above specific Y values in Plotly. This technique helps highlight important data ranges and thresholds in your visualizations. Combine with reference lines for better clarity.