One of the ways of using a line chart is for converting values between different measurement scales.
For example, in old recipe books, you may sometimes see conversion charts that have an imperial measurement (e.g. ounces) on one axis, and a metric quantity described on the other.
The chart line then plots out equivalent values on each scale.
To find out more about using line charts in this way, read all of Section 3 Graphical conversions from the the OpenLearn unit Exploring distance time graphs.