5.2.3 Graphs of Time Series or Line Graphs
When we observe the values of a variable at different points of time, the series so formed is known as time series. The technique of graphic presentation is extremely helpful in analysing change at different points of time. On the X-axis we generally take the time and on the Y-axis the value of the variable and join the various points by straight lines. The graph so formed is known as the line graph. Such graphs are most widely used in practice. They are the simplest to understand, easiest to make and most adaptable to many uses. They require the least technical skill and at the same time enable one to present more information of a complex nature in a perfectly understandable form than any other kind of chart. Many variables can be shown on the same graph and a comparison can be made.
Graphs of time series can be constructed either on a natural scale or on a ratio scale. In natural arithmetic scale absolute change from one period to another area shown whereas in a ratio scale the rates of change or the relative changes are shown. First of all, we take up line graphs on a natural scale and then study such graphs on ratio scale.