How to Make One Trendline for Multiple Series in Excel

Adding a trendline to a chart in Excel is usually simple, but trying to stretch a single trendline across multiple data series can be confusing. Excel's default behavior is to treat each series independently, which is great for comparisons but not when you need to see the single, overarching trend for your whole dataset. This guide will walk you through a reliable workaround to create one master trendline for all your data series.

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Why Single Trendlines for Multiple Series Can Be Tricky

By default, Excel assumes that if you have multiple series on a chart - say, sales for Product A, Product B, and Product C - you want to analyze their trends separately. When you right-click and add a trendline, Excel applies it to whichever series you clicked on. This is logical and often useful, as it lets you see if Product A is trending up while Product C is flat.

However, there are many times you need to understand the big picture. Are your sales, as a whole, trending up or down? Is there an overall pattern when you combine all user activity, not just from one channel? Creating this kind of “master trendline” requires giving Excel a single, combined data source to analyze, and that’s exactly what the following method accomplishes.

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The “Helper Series” Method: The Easiest Way to Create a Master Trendline

The solution is to create an intermediate, "helper" data series that pools all your data points together. We'll add this invisible series to the chart and then have Excel draw a trendline based on it. The original data series will remain visible and untouched, giving you the best of both worlds: individual series plus an overall trend.

Step 1: Organize Your Data

First, make sure your data is set up in a standard table format. You should have one column for your X-axis labels (like dates or months) and separate columns for each Y-axis data series.

For this example, let's say we're tracking quarterly website traffic from two sources: Organic Search and Paid Social.

Your table should look something like this:

Step 2: Create Your Initial Chart

Now, let's create a chart from this data so we have a visual starting point.

• Select your entire data table (from Quarter to the last data point).
• Go to the Insert tab in the ribbon.
• In the Charts section, click on Insert Scatter (X, Y) or Bubble Chart and choose the Scatter with Straight Lines and Markers option. A scatter plot is often best for trend analysis because it properly maps each Y point to its corresponding X value.

Your chart will appear showing two distinct, color-coded lines: one for Organic Search and one for Paid Social. If you tried to add a trendline now, Excel would force you to choose one series or the other.

Step 3: Prepare the Helper Data

This is where the magic happens. We need to combine all our X and Y values into two long columns. This essentially "unstacks" your data so it behaves like a single series.

Find some empty space in your worksheet and create two new columns: Helper X and Helper Y.

• Under Helper X, copy and paste the Quarter column from your original data.
• Then, paste the entire Quarter column again directly below the first set. Your helper X-column should now be twice as long as the original.
• Under Helper Y, copy the Organic Search data and paste it into the top half, next to the first set of quarters.
• Next, copy the Paid Social data and paste it into the bottom half, next to the second set of quarters.

Your helper data structure should now look like this:

This new table aggregates all individual data points into a single pairing of X and Y values that Excel can analyze as one group.

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Step 4: Add the Helper Series to Your Chart

Next, we will add this combined data to our existing chart as a new, third series.

• Right-click anywhere on your chart and choose Select Data.
• In the Select Data Source window, under Legend Entries (Series), click the Add button.
• An Edit Series dialog box will open:
• Click OK and then OK again.

Your chart will now look a bit crowded. You'll see the two original lines plus a new set of scattered points representing the combined data.

Step 5: Add the Trendline to the Helper Series

Now, we can finally add the master trendline based on our hidden data.

• Find one of the new data points from the helper series you just added and single-click it to select the entire series.
• Right-click on one of the selected points and choose Add Trendline... from the menu.

A single, dotted trendline will appear across your chart, representing the overall trend of both Organic Search and Paid Social data combined. In the Format Trendline pane that appears on the right, you can customize its appearance (color, dash type) and even display the equation or R-squared value for a deeper analysis.

Step 6: Make the Helper Series Invisible

The final step is to clean up the chart by hiding the scattered helper points, leaving only the beautiful master trendline.

• Right-click on one of the helper data points again and choose Format Data Series....
• In the Format Data Series pane, click on the paint bucket icon labeled Fill & Line.
• Under the Line settings, select No line.
• Under the Marker settings, expand the Marker Options and select None.

The helper data points will disappear from the chart, but the trendline remains! You're left with a clean chart showing your original two series and a single overarching trendline.

For an extra touch, you can also click on the legend entry for your helper series (e.g., "Overall Trend Data") and press the Delete key. This removes it from the legend without affecting the chart, making everything look intentional and professional.

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An Alternative: The "Total" or "Average" Trendline

Sometimes, what you're really looking for is not a trend of every single combined data point, but rather the trend of the total or average value at each time interval. This approach is much simpler.

1. Create a Calculation Column: In your original data table, add a new column called "Average Traffic" or "Total Traffic."
2. Enter a Formula: In the first cell of the new column, enter a formula like =AVERAGE(B2,C2) (for the average) or =SUM(B2:C2) (for the total). Drag this formula down to apply it to all rows.
3. Add the New Series: Right-click your chart, choose Select Data, and add this new "Average/Total Traffic" column as another series.
4. Apply the Trendline: Right-click on the new line that appears on your chart (representing the average or total) and select Add Trendline.

This method gives you a trend based on the average or sum's performance over time, which can sometimes be a clearer way to see top-level movement instead of combining all the underlying data. You can then choose to keep the average/total line visible or make it invisible using the formatting steps above.

Final Thoughts

While Excel doesn’t offer a one-click solution for adding a trendline across multiple series, the helper series method is a flexible and powerful workaround. By manually combining your data into a single hidden series, you can easily create clean, insightful charts that show both individual performance and the overall trend that ties it all together.

Of course, building helper columns and spending time formatting charts is a familiar part of getting insights from spreadsheets. We created Graphed because we believe your time is better spent acting on data than wrestling with it. Instead of manually preparing data in Excel, you can connect your sources like Google Analytics or marketing platforms and get custom dashboards by simply describing what you want. For instance, asking "Show me a chart of my overall website traffic trend, segmented by paid and organic sources" gets you an interactive dashboard, instantly and with no manual setup required.