You can show trend lines in a visualization to highlight trends in your data.
To add trend lines to a view, both axes must contain a field that can be interpreted as a number. For example, you cannot add a trend line to a view that has the Product Category dimension, which contains strings, on the Columns shelf and the Profit measure on the Rows shelf.
However, you can add a trend line to a view of sales over time because both sales and time can be interpreted as numeric values.