An Excel trendline represents the general direction of data rather than connecting every actual observation. It can be straight or curved depending on the chosen model. The guide describes this as a useful way to inspect movements over time or relationships between variables.

Use a Trendline to Show a Chart's Overall Pattern

Trendlines are available only for suitable chart types; stacked and three-dimensional charts are among the unsupported cases discussed. Choose a model that fits the analytical question and examine the observations as well as the fitted line. A smooth visual pattern should not be interpreted as a guarantee that future values will follow it.