Add a Trendline to an Excel Combo Chart: Full Guide

To add a trendline to an Excel combo chart in Microsoft 365 for Windows, select the chart, click the data series you want to analyze, then choose Chart Design > Add Chart Element > Trendline and select a type, such as Linear. A trendline belongs to one data series at a time; it does not calculate a single trend from every series in the combo chart.

How trendlines work in a combo chart

A combo chart displays two or more data series using different chart types, commonly columns for totals and a line for a rate or target. A trendline is a statistical fit to the values in one series. For example, a chart might show monthly sales as columns and profit margin as a line. You can add a trendline to sales or to margin, but each line answers a different question and should be added separately if you want to show both.

Excel fits the trendline to the selected series’ data. It does not simply connect the first and last points, and it does not combine the column series with the line series. The original values remain unchanged; the trendline is a visual chart element. Its equation and R-squared value can also be shown, but those are optional and are not automatically displayed.

Trendlines work with many common two-dimensional series, including line, column, bar, and scatter series. Certain chart types, such as 3-D, pie, and doughnut charts, do not support them, and some specialized or stacked chart configurations may limit the available options. If Excel disables the command, check the selected series and its chart type before rebuilding the chart.

Check the chart and data before adding one

Before changing the chart, decide which series has a meaningful trend. A trendline is most useful when the observations have a sensible order, such as dates, production runs, or increasing quantities. A line through unrelated categories can look persuasive while implying a progression that the data does not support.

Confirm the selected series. Click a column or a point on the line, not just the chart background. The whole series should become selected.
Check the horizontal axis. Make sure dates or values are sorted in the intended order. If dates are stored as text, Excel may treat them as labels rather than date values.
Review the chart type. A standard two-dimensional line-and-column combo chart is a practical starting point. Remove unsupported chart types if the trendline controls are unavailable.
Look for missing or unusual values. Blank cells, zeros used to represent missing data, and extreme outliers can change the apparent fit.

For example, a monthly chart with revenue columns and a customer-count line can show a trend for either measure. If the customer count is on a secondary axis, decide whether its scale and trend are easier to read than those of revenue before adding another line.

Add a trendline in Excel for Windows

Use the chart ribbon when you want to be certain which series is selected and need access to the full set of trendline settings.

Click the combo chart once. Then click the specific series you want to analyze. If necessary, click one of its columns or line markers again so Excel selects the series rather than the chart area.
Open Chart Design > Add Chart Element > Trendline.
Choose a quick option, such as Linear, or select More Trendline Options to open the formatting pane.
In the pane, choose the trendline type and adjust any relevant settings. For a linear fit, you can choose whether to display the equation or the R-squared value on the chart.
Close the pane and review the result. Check that the trendline is associated with the intended series and is readable against the chart’s axes.

You can also right-click a data point in the series and select Add Trendline. This is quick, but confirm that the series you meant to select is highlighted. To remove the line, select the trendline and press Delete, or use the chart element controls to turn it off. Removing a trendline does not delete the underlying worksheet data.

Choose a trendline type that fits the question

Excel offers several models. The right choice depends on the pattern you are trying to describe, not on which option makes the chart look smoothest.

Linear: Fits a straight line and is a reasonable first look when values rise or fall at roughly a steady rate. For instance, it can summarize weekly output that increases by a similar amount each week.
Exponential: Models a rate of increase or decrease that compounds. It is not suitable when the series includes zero or negative values.
Logarithmic: Can represent rapid change that slows over time. It requires suitable positive input values and should only be used when that shape makes sense for the data.
Polynomial: Fits a curve with one or more bends. Excel lets you set its order; a higher order can follow more wiggles, but may overfit a small data set.
Power: Models a power relationship and requires positive values. Use it only when the relationship has a defensible reason to follow that form.
Moving Average: Smooths short-term variation by averaging a chosen number of neighboring points. It can make a noisy series easier to inspect, but it is not the same as a fitted equation.

Start with a linear fit when the purpose is simply to check direction, then compare alternatives only if the data pattern or subject matter supports them. A curved line can fit historical points closely without being a reliable forecast.

Working with a secondary axis

Combo charts often put a series with a different scale on a secondary vertical axis. For example, revenue might range from hundreds to thousands while a percentage stays between zero and one hundred. A secondary axis makes both series visible, but it can also make their slopes look more alike or different than they are.

A trendline is tied to the series from which it was created. If you add one to a series plotted on the secondary axis, check that it appears against the expected scale. Use the chart’s axis labels and legend to keep the relationship clear, and avoid changing axis limits merely to make a trendline appear steeper or flatter. Two series with different units should not be compared by visual slope alone.

If the axis assignment is wrong, select the series, open Format Data Series, and use Series Options to choose the primary or secondary axis. The wording and pane layout can vary slightly with the Excel build, but the axis choice is available from the series-formatting controls. Recheck the trendline after changing the series axis because its visual position is interpreted using that axis’ scale.

Read the equation, R-squared, and forecast settings carefully

In More Trendline Options, Excel can display a fitted equation and, for supported trendline types, an R-squared value. The equation describes the fitted relationship in chart coordinates; it is not automatically a formula for the original worksheet cells. In charts with category labels, the horizontal input may effectively represent category positions rather than the actual spacing between dates or labels. That distinction matters if the intervals are irregular.

R-squared indicates how closely a fitted model accounts for variation in the data used for that fit. A value near one means a closer fit to those plotted points, not proof that the model is correct, that one variable causes another, or that future values will follow the line. A complex polynomial can produce a high value simply because it bends around the observed points.

The forecast controls extend a trendline forward or backward by a specified number of units. Treat this as a visual projection, not a promise or a complete forecasting method. In a category chart, the units may correspond to data points or category positions rather than calendar time. If accurate spacing between numeric x-values or dates is important, consider an XY scatter chart with a numeric horizontal axis.

Common problems and practical fixes

Trendline is missing or unavailable: Select an individual supported series, not the entire chart. If the command remains disabled, check whether that series uses an unsupported chart type or whether the chart is three-dimensional or stacked.
The line appears on the wrong data: Undo or delete it, select a point in the intended series, and add the trendline again. In a combo chart, clicking a nearby column or marker can select a different series than expected.
The line looks flat or unusually steep: Inspect the vertical-axis scale and whether the series uses the secondary axis. A scale difference can strongly affect visual slope without changing the original data.
The trendline does not follow calendar spacing: A line chart normally treats the horizontal axis as categories. For uneven time gaps or numeric x-values, use an XY scatter chart where x-values are plotted according to their numeric distance.
An exponential or power option fails: Check for zeros or negative values; these models require suitable positive data. Choose another model if the data cannot meet that requirement.
The chart is cluttered: Remove unnecessary trendlines, equation labels, or markers. Use a clear legend or direct series labels so readers can tell which data the trendline describes.

In Excel for Mac, select the series and look for the chart-design controls or the contextual menu to add a trendline; the ribbon layout may differ by build. Excel for the web has fewer chart-formatting controls than the Windows desktop app. If the trendline command or a needed model setting is not available there, open the workbook in desktop Excel rather than assuming the chart itself is broken.

When a trendline is not the best choice

A trendline is helpful for summarizing a pattern, but it is not always the clearest way to communicate change. If the reader needs to compare actual values, the original columns or line may be enough. If the goal is to show a smoothed path through noisy observations, a moving-average trendline may help, though it can lag behind a sudden change.

For a genuine time-series forecast, consider using a method that accounts for seasonality and changing patterns rather than extending a simple chart line. Excel’s forecast worksheet tools or a dedicated statistical method may be more appropriate, depending on the question and the data. Whatever method you use, label historical observations and projections clearly, state the assumptions, and avoid presenting a fitted line as a causal explanation.

Frequently Asked Questions

Can I add a trendline to both series in a combo chart?
Yes, if both series and their chart types support trendlines. Add the first line to one series, then select the other series and repeat the steps. Each trendline fits only its own series.

Can a combo chart have a trendline on a column series?
Often, yes. Select the column series itself and add the trendline. If the option is unavailable, check the series’ chart type and whether the chart uses a configuration that restricts trendlines.

Why is Add Trendline greyed out?
The chart or selected series may not support a trendline, or you may have selected the chart area instead of a data series. Try selecting a single point in a supported two-dimensional series and check the chart type.

Can I add a trendline in Excel for the web?
Some chart editing is available in Excel for the web, but its controls can be more limited than desktop Excel. If you cannot find the trendline command or required options, open the workbook in Excel for Windows or Mac.

Does a high R-squared prove that the trend will continue?
No. It describes how well the selected model fits the observed data, not whether the model explains the cause or predicts future values accurately. Consider the data, assumptions, and context before treating a projection as meaningful.

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Sources: Function behaviour and menu paths: Microsoft Support documentation for Excel. Method & sources

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