How to plot on secondary Y-Axis with Python Plotly?

Plotly is an open-source, interactive, and browser-based charting library for Python. Python users can use Plotly to generate different types of charts including scientific charts, 3D graphs, statistical charts, financial charts, etc.

In this tutorial, we will show how you can use Plotly to plot data on the secondary Y-Axis. Here we will use the plotly.graph_objects module to generate figures. It contains a lot of methods to customize the charts and render them into HTML format. We will plot two bar charts using the add_trace() method and then use the update_layout() method to set a property with dict arguments.

Step-by-Step Implementation

Step 1: Import Required Module

Import the plotly.graph_objects module and alias as go ?

import plotly.graph_objects as go

Step 2: Create a Figure

Create a figure using the Figure() method ?

import plotly.graph_objects as go

fig = go.Figure()
print("Figure created successfully")

Figure created successfully

Step 3: Add Bar Charts with Different Y-Axes

Create two bar charts using the add_trace() method. The first chart uses the primary Y-axis (y1) and the second uses the secondary Y-axis (y2) ?

import plotly.graph_objects as go

fig = go.Figure()

# Add first bar chart (primary Y-axis)
fig.add_trace(go.Bar(
    x=[5, 6, 7],
    y=[1, 2, 3],
    name="Primary Axis",
    yaxis='y'
))

# Add second bar chart (secondary Y-axis) 
fig.add_trace(go.Bar(
    x=[1, 2, 3],
    y=[100, 200, 300],
    name="Secondary Axis",
    yaxis="y2"
))

print("Bar charts added successfully")

Bar charts added successfully

Step 4: Configure the Axes

Create axis objects for the first and second Y-axis using update_layout() ?

import plotly.graph_objects as go

fig = go.Figure()

fig.add_trace(go.Bar(
    x=[5, 6, 7],
    y=[1, 2, 3],
    name="Primary Axis",
    yaxis='y'
))

fig.add_trace(go.Bar(
    x=[1, 2, 3],
    y=[100, 200, 300],
    name="Secondary Axis",
    yaxis="y2"
))

# Configure the axes
fig.update_layout(
    # Primary Y-axis configuration
    yaxis=dict(
        title="Primary Y-axis",
        titlefont=dict(color="blue"),
        tickfont=dict(color="blue")
    ),
    
    # Secondary Y-axis configuration
    yaxis2=dict(
        title="Secondary Y-axis", 
        overlaying="y",
        side="right",
        titlefont=dict(color="red"),
        tickfont=dict(color="red")
    )
)

print("Axes configured successfully")

Axes configured successfully

Complete Example

Here is the complete code to plot on secondary Y-axis ?

import plotly.graph_objects as go

# Create figure
fig = go.Figure()

# Add first bar chart (primary Y-axis)
fig.add_trace(go.Bar(
    x=['A', 'B', 'C'],
    y=[1, 2, 3],
    name="Sales (Units)",
    yaxis='y',
    marker_color='lightblue'
))

# Add second bar chart (secondary Y-axis)
fig.add_trace(go.Bar(
    x=['A', 'B', 'C'], 
    y=[100, 200, 300],
    name="Revenue ($)",
    yaxis="y2",
    marker_color='lightcoral'
))

# Configure layout with dual Y-axes
fig.update_layout(
    title="Dual Y-Axis Bar Chart",
    
    # Primary Y-axis (left side)
    yaxis=dict(
        title="Units Sold",
        titlefont=dict(color="blue"),
        tickfont=dict(color="blue")
    ),
    
    # Secondary Y-axis (right side)
    yaxis2=dict(
        title="Revenue ($)",
        overlaying="y",
        side="right", 
        titlefont=dict(color="red"),
        tickfont=dict(color="red")
    ),
    
    # X-axis configuration
    xaxis=dict(title="Products"),
    
    # Layout styling
    width=700,
    height=400,
    showlegend=True
)

# Display the chart
fig.show()

Key Parameters

Conclusion

Secondary Y-axes in Plotly allow you to display datasets with different scales on the same chart. Use yaxis2 with overlaying="y" and side="right" to create the secondary axis, making it perfect for comparing related metrics like units and revenue.