Why an Excel Polynomial Trendline Formula Doesn't Match the Graph
Question details
The user's Excel polynomial trendline equation yields results that do not align with the visually plotted graph when calculating fitted values.
- Product
- Excel
- Device & OS
- not provided
- Scenario
- Using a polynomial trendline equation with large x-values (such as four-digit years) to calculate fitted values in a spreadsheet.
- Observed behavior
- The formula produces results that are significantly different from the plotted trendline on the chart due to numerical precision limitations when Excel calculates polynomial coefficients for large x-values.
Verify if your x-axis values contain large numbers (like 2005, 2019) and ensure your dataset is properly sorted before attempting to adjust the trendline calculations.
Convert Large X-Values to Small Year Offsets
Transforming large year numbers into smaller numerical offsets (e.g., 0, 1, 2) eliminates numerical precision errors when Excel calculates complex polynomial coefficients.
Large x-values combined with higher-degree polynomials exceed standard calculation limits. Because polynomial calculations involve raising x-values to the 2nd, 3rd, or higher powers, values like 2019^3 create massive numbers that force Excel to truncate coefficients, causing severe inaccuracies when plugged back into the formula.
Insert a new column next to your original x-axis values (years).
In the new column, assign a small starting number to your earliest year, such as 0 or 1 (e.g., for the year 2005, type 0).
Calculate the following years as incremental offsets from your base year (e.g., 2006 becomes 1, 2019 becomes 14).
Select the new offset column as your x-axis data, re-plot your chart, and generate the polynomial trendline equation. The resulting coefficients will now produce accurate results.
Use Worksheet Functions (SERIESSUM) for Fitted Values
Instead of relying on the chart's trendline label, use advanced worksheet formulas to calculate accurate fitted values directly in your spreadsheet.
Create and Manage Polynomial Trendlines with WPS Spreadsheet
WPS Spreadsheet provides powerful and precise charting tools to easily plot data, add polynomial trendlines, and calculate accurate formulas for complex data analysis without precision loss.
- 1. Import Your Data: Open your data file in WPS Spreadsheet. The software automatically supports all standard Excel formats.
- 2. Create a Scatter Chart: Highlight your data range and navigate to 'Insert' > 'Chart' to create a scatter plot.
- 3. Insert a Trendline: Right-click the plotted data series on the chart and select 'Add Trendline' from the context menu.
- 4. Configure the Polynomial Equation: Choose the 'Polynomial' type, select your desired order (degree), and check 'Display Equation on chart' to view the formula.
Frequently Asked Questions
Why does the trendline look correct on the chart but the formula gives wrong answers?
Spreadsheet software rounds the coefficients displayed in the chart's visual label for readability. For higher-degree polynomials with large x-values, this visual rounding causes massive precision loss when you manually plug those rounded numbers back into a worksheet formula.
Can I increase the decimal precision of the trendline formula on the chart?
Yes. You can click on the trendline equation label on the chart, open the format properties (Format Trendline Label), change the number category to 'Number' or 'Scientific', and increase the decimal places to 10 or more to improve formula accuracy.
Does this calculation issue only happen with polynomial trendlines?
While precision issues are most pronounced and problematic with polynomial trendlines (especially 3rd degree or higher), exponential and power trendlines can also experience minor calculation discrepancies when used against extremely large raw datasets.
What is the maximum polynomial degree supported in spreadsheet charts?
Most major spreadsheet programs, including WPS Spreadsheet and Microsoft Excel, support up to a 6th-order (degree) polynomial trendline.
