A time series plot is a graph of observations against time. Time is shown on the x-axis and the observed value is shown on the y-axis. The points are joined in chronological order so that the path of the series can be followed.
A time series plot is a graph of observations against time. Time is shown on the x-axis and the observed value is shown on the y-axis. The points are joined in chronological order so that the path of the series can be followed.
Monthly shop sales from January to December are plotted with months along the x-axis and sales along the y-axis. January comes before February; the order is never shuffled by size.
A line plot emphasises sequence. Neighbouring months or days are connected, so a rise, fall or repeating wave is easier to see than in a bar chart of unsorted values. Bars can still be used for a few discrete periods, but the standard teaching display is a line through time-ordered points.
| Appearance | Possible meaning | Simple example |
|---|---|---|
| Rising line | Values tend to increase over time | Sales growing through the year |
| Falling line | Values tend to decrease over time | Demand falling after a peak season |
| Repeating pattern | A similar shape returns at a regular interval | Higher sales every December |
| Sudden spike or drop | An unusual observation or a short shock | One week of very high festival sales |
A rising line does not prove that next month must be higher. It only describes what has happened so far. A spike should be studied: it may be a data error, a one-time event, or the start of a new level.
Suppose monthly sales run roughly: low in February, higher towards December, and generally a little higher than the previous year. The plot would show a repeating yearly wave sitting on a gentle upward movement. That picture is the starting point for later smoothing, numerical summary and modelling.
The exact height of each point depends on the dataset. Students should describe the shape of their own plot rather than memorising invented numbers.
The following snippet only shows how a chronological plot is prepared. Run it outside the portal. The chart should show eight weeks in time order with a general rise and one small dip.