Time is the independent variable. Often the observations are equally spaced over time. Because the order of the observations is important we want graphical techniques that emphasize the ordering. The following sets of graphs show a time series of the annual number of lynx trappings in the Mackenzie River District of North-West Canada for the period 1821 to 1934. This is just the usual scatter plot.
Time is on the horizontal axis.
This often fails to show the sequential nature of the data. This is a graph where sequential data points are connected by lines but no special symbol is drawn at the individual observations. This graph shows the sequential nature of the data but we often cannot distinguish between a pattern in the points and an outlier. This is a combination of the above two graphs. A symbol is drawn at each point and then connected by lines.