Multiple Line Graphs — Fundamental Concepts

Fundamental Concepts

Identify the series and time scale

Read the legend and both axes. Establish whether the vertical values are quantities, percentages or index numbers. Check the actual time interval between observations; adjacent plotted points may be two years apart in one section and one year apart in another.

Separate level from movement

Suppose series A moves from 120 to 132 while B moves from 60 to 72 over the same year. Both increase by 12, but A grows by 10% and B by 20%. A remains at a higher level while B has the higher percentage growth. These statements are compatible.

To compare average change per year, divide the change by the elapsed years. A rise of 30 across three years averages 10 per year. It should not be ranked as faster than a rise of 16 across one year merely because its total change is larger.

Avoid unsupported interpolation

If observations are available only in January and March, the connecting line is a visual link. It does not establish February's actual value unless the problem states linear movement or asks for a linear estimate. Likewise, an apparent crossing between dates need not identify an exact observed equality date.

For index series, the base period and original scale matter. Two series both starting at 100 have equal index values, not necessarily equal physical quantities. Use the graph to answer the metric it actually reports.