Line charts exist to show change across an ordered scale, and time is by far the most common one.
The line connects neighbouring points, so the eye reads the direction and speed of change rather than single values.
A steeper slope means faster change, and a turning point shows where the direction reversed.
Several lines on one chart compare trends, such as this year's sales against last year's, month by month.
Beyond 4 or 5 lines the chart becomes tangled, and highlighting one line while greying the rest helps.
The value axis may start above 0 when the size of the changes, not the level, is the message.
Gaps in the data should show as gaps in the line, because joining across missing months invents values.
Uneven spacing is a trap: if years 4 apart sit as close as years 1 apart, every slope is wrong.
Marking key events, such as a launch, directly on the line explains jumps without a separate note.
Noisy daily data makes a line look jagged, and readers may read meaning into random wiggles.
Stretching or squeezing the axis range can make a small change look dramatic or a big one look flat.
Lines of very different size compare better as an index, with each line starting at 100.
Area charts fill the space under a line, which stresses totals but makes overlapping series hard to read.
In practice, line charts are the standard for any measure watched over time, from sales to temperatures.
The usual mistake is joining points that have no order, such as regions, which suggests a trend that does not exist.