The line mark type is useful when you want to see trends in data over time, your data are ordered, or interpolation makes sense. Tableau displays data using lines when:
The Mark menu is set to Automatic,
and you place one or more measures on either the Columns shelf
or the Rows shelf, and then plot the measures
against a date dimension or a continuous dimension.
You select Line from the Mark menu.
With the line mark type, you can specify the drawing order of the line by placing a field on Path.
As the density of data increases, trends are often easier to see when using lines. This view shows 93 data points.
To try some hands-on exercises for building line charts, see Building Line Charts.