To fit a line on an Excel plot to a portion of the data, you can use the trendline feature. Here are the steps:

1. Select the data points that you want to include in the trendline.
2. Right-click on one of the selected data points and choose "Add Trendline" from the context menu.
3. In the "Format Trendline" pane that appears on the right side of the screen, select the "Linear" option under "Trendline Options".
4. Under "Forecast", you can choose the number of periods to forecast into the future. Leave this at 0 if you don't need to forecast.
5. Under "Line", you can choose the color and style of the trendline.
6. Under "Options", you can choose to display the equation and R-squared value on the chart.
7. To limit the trendline to a portion of the data, you can use the "Set Intercept" and "Set Slope" options under "Customize Trendline".
8. Adjust the "Set Intercept" and "Set Slope" values until the trendline passes through the desired portion of the data.

By following these steps, you should be able to fit a best fit line to a portion of the data in Excel.