In general, the different observations are correlated , that is the value of the time series at time t n depends on the value at time t n-1 .
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.