Time-ordering of univariate data
Some data sets may appear to be univariate, but the measurements are made sequentially in time. For example, many demographic and economic measurements are recorded monthly or annually. A data set of this form is called a time series.
Treating time as an additional numerical variable allows the data to be treated as bivariate. In particular, a scatterplot of the variable against time is called a time series plot.
Time series data can be displayed and analysed with many of the techniques that are used for bivariate data.
Gross Domestic Product in the USA