**Excel** does not natively allow you to add a sine wave as a trendline (the built-in trendlines are linear, polynomial, exponential, etc.), but **you can fit a sine wave to your data and plot it alongside your data points.** Here’s how you can do it: --- ## **1. Fit a Sine Curve to Your Data** A general sine function is: ``` y = A * SIN(B * x + C) + D ``` Where: - **A** = amplitude - **B** = frequency (related to period) - **C** = phase shift - **D** = vertical offset **The steps:** ### **a. Estimate Parameters** - **A**: Half the distance between your maximum and minimum amplitude values. - **D**: The average (mean) of your amplitude values. - **B**: `2π / period`, where period is the distance it takes for the wave to repeat (can estimate by looking at your data). - **C**: The phase shift needed to align with your data (may require trial and error). ### **b. Create a 'Fitted Sine' Column** - In a new column, use the formula: `=A * SIN(B * [Distance] + C) + D` - Replace A, B, C, D with cell references or fixed values. --- ## **2. Use Solver for Better Fit (Optional)** If you want Excel to find the best fit: 1. Put A, B, C, D in dedicated cells. 2. In a new column, calculate the fitted sine using your formula and those parameter cells. 3. In another cell, calculate the sum of squared differences between your data and the sine fit. 4. Use **Data > Solver** to minimize this sum by changing A, B, C, D. --- ## **3. Plot Your Data and the Sine Fit** - Plot your original data (scatter plot). - Add your fitted sine values as a new series (line). - This overlays the sine "trendline" on your scatter plot. --- ### **Summary** **Excel can’t add a sine trendline automatically, but you can manually fit and plot a sine wave.** If you want a step-by-step example with formulas, let me know your data range and I'll write it out!