How to Overlay Two Graphs in Tableau

Combining two different charts into one provides a powerful way to compare trends and uncover relationships in your data. In Tableau, the go-to method for this is creating a dual-axis chart, which lets you overlay two graphs by giving each its own axis. This article walks you through exactly how to build, customize, and effectively use dual-axis charts to compare different measures in a single, insightful visualization.

What Exactly is a Dual-Axis Chart?

A dual-axis chart is a single chart that displays two different measures against the same dimension. Think of it as laying one transparent chart on top of another. The magic is that each measure gets its own axis, allowing you to plot data with completely different scales. For instance, you could show a company’s revenue (measured in millions of dollars) and its market share (measured as a percentage) over time in the same graph.

This is incredibly useful for several reasons:

• Comparing variables with different units: You can easily compare metrics like website traffic (a whole number) against your conversion rate (a percentage), or your ad spend (currency) against your click-through rate (percentage).
• Showing relationships between measures: Does your profit increase when your sales go up? Does your website conversion rate drop when you get a spike in traffic from a new source? A dual-axis chart makes these potential correlations immediately visible.
• Saving dashboard space: Instead of creating two separate charts to show two related measures, you can combine them into one, making your dashboards cleaner and more efficient.

The most common and effective use case is a combination chart, where one measure is represented as bars and the other as a line. This provides a clear visual distinction between a volume-based metric (like sales or website sessions) and a rate-or-ratio-based metric (like profit margin or conversion rate).

Step-by-Step: Creating Your First Dual-Axis Chart

Let’s build a classic dual-axis chart to compare Sales and Profit over time. This is a perfect example because both are measured in dollars, but the scales are often quite different.

We'll assume you've already connected to your data source (like the Sample - Superstore dataset that comes with Tableau).

Step 1: Build Your First Chart

First, create a simple line chart for your primary measure. This will be the foundation of our dual-axis chart.

1. Drag your time dimension, like Order Date, onto the Columns shelf.
2. Right-click the Order Date pill in the Columns shelf and select Month to view the data on a monthly basis. Choose the continuous month option (the one with the green calendar icon) for a smooth time-series line.
3. Drag your first measure, Sales, onto the Rows shelf.

At this point, you should see a simple line chart showing total sales for each month.

Step 2: Add Your Second Measure

Now, let's bring in the second measure that you want to compare — in this case, Profit.

1. Find your second measure, Profit, in the Data pane.
2. Drag Profit onto the Rows shelf and place it to the right of the existing SUM(Sales) pill.

Tableau will now display two separate line charts, one stacked on top of the other, each with its own axis on the left.

Step 3: Combine the Graphs with Dual Axis

This is where the magic happens. We'll tell Tableau to merge these two separate charts into one.

• Right-click the second measure pill on the Rows shelf — in our case, SUM(Profit).
• From the dropdown menu, select Dual Axis.

Just like that, Tableau overlays the two line charts. You’ll now see the Sales axis on the left and a new Profit axis on the right. Your two measures are now plotted in the same space, but referenced against their own independent scales.

Customizing and Refining Your Dual-Axis Graph

Creating the initial chart is easy, but a few simple tweaks can make it a lot more readable and effective.

Changing the Mark Type (e.g., Line and Bar Chart Combo)

One of the most effective ways to visualize two measures is by using different chart types for each. For instance, showing sales as bars and profit margin as a line. Let's adjust our current Sales vs. Profit chart.

Under the Marks card on the left, you’ll now see tabs for each of your measures (SUM(Sales) and SUM(Profit)), plus one for All.

1. Click on the SUM(Sales) Marks card.
2. Click the dropdown menu that likely says “Automatic” and select Bar.
3. Now, click on the SUM(Profit) Marks card.
4. Change its mark type to Line. You can also adjust the colors here to make the line stand out from the bars.

You now have a combination chart that clearly distinguishes volumes (Sales) from a performance outcome (Profit). This is much easier to read than two overlapping lines.

Synchronizing the Axes

This is a critical step that many people overlook. By default, the two axes in a dual-axis chart are independent. This is perfect when you're comparing fundamentally different scales, like dollars and percentages. However, if your two measures share the same unit (like our Sales vs. Profit example), you should often synchronize the axes.

Why? Without synchronized axes, the visual comparison can be misleading. A slight dip in a line on an unsynchronized axis could look bigger than a massive drop on the primary axis. Synchronizing them ensures an "apples-to-apples" comparison.

To synchronize your axes:

• Right-click on either of the Y-axes in your chart (the Profit axis on the right, for example).
• Select Synchronize Axis.

Now, both axes will use the same scale, giving you a true representation of the relationship between the two measures. You'll instantly see months where profit was high relative to sales. In our Sales and Profit chart, we can now clearly see if there were any months that resulted in a loss when sales were also down.

Best Practices for Effective Dual-Axis Charts

Dual-axis charts are powerful, but they can also be confusing if not designed carefully. Follow these tips to keep your visuals clear and honest.

• Use Distinct Colors and Shapes: Your audience needs to know which line or set of bars belongs to which axis. Use contrasting colors that are easy to distinguish. If you're using two line charts, consider making one dashed or using different shape markers for the data points.
• Label Everything Clearly: Don't make people guess. Use a clear chart title that describes what's being compared (e.g., "Monthly Sales Volume vs. Profit Ratio"). Ensure both axes are labeled with the measure name and unit. A color-coded legend is also essential.
• Don't Distort the Narrative: Be cautious about using dual-axis charts to compare two loosely related metrics. Plotting "Number of Social Media Mentions" against "Company Revenue" might create a visual correlation that isn't statistically real. Just because two lines trend together doesn't prove causation. Use this chart type to explore relationships, not to force them.
• Know When Not to Use It: If the two measures you're comparing are too difficult to analyze together, or if there's no meaningful relationship between them, it's better to just show them in two separate charts side-by-side on your dashboard. Simplicity is often better than a cluttered, confusing combination.

Final Thoughts

Overlaying graphs in Tableau using the dual-axis feature is a fundamental skill for anyone looking to build more advanced and insightful dashboards. It allows you to visualize direct comparisons between different metrics, revealing trends and correlations that might otherwise remain hidden by displaying them in a clear, consolidated view.

With tools like Tableau, you can build powerful custom reports, but it often requires a learning curve and manual effort to get them just right. At Graphed , we're focused on streamlining this entire process. Instead of dragging and dropping pills and clicking through menus, you can connect your data sources (like Google Analytics, HubSpot, or Shopify) and simply ask for the chart you want in plain English. We turn hours of report-building into a simple conversation, so you can focus on the insights, not the setup.