The trendline fits across your data.
You add a trendline to your chart by clicking the chart and then, on the Chart menu, clicking Add trendline. You need to choose what type of trendline you want: linear, logarithmic, polynomial, power, exponential, exponential or moving average.
In this case, you know your data should be linear, so choose that. You may notice that some of the other options are unavailable; this is because the chart type doesn't support that type of curve, so Excel isn't giving you those options.
You can choose to show the equation of the curve and R² value on the chart too. You'll see how to do this in the practice session.
The following types of charts support trendlines:
- XY scatter