A chart begins at 100 and later reaches 125. That does not mean a product costs 125 pounds or that 125 people were counted. The chart may be an index expressing change relative to a selected reference period.
The Office for National Statistics’ explanation of consumer-price outputs describes indices presented with a reference period equal to 100. Read the series title and methodological notes to establish what is being measured and which period supplies that reference.
Work through a small example
Suppose an invented series moves from 100 to 120, then to 132. The first change is 20%. The second is 10%, because the increase of 12 is divided by 120. Adding twelve index points is not the same as rising twelve percent from every starting value.
If the same series is simply rescaled so that its middle observation becomes 100, the three values become approximately 83.33, 100 and 110. The displayed levels change, while the percentage movements between those observations remain the same.
Do not compare unrelated levels as quantities
Two industries can both start at 100 even if one is much larger. Their indexed lines show relative movement from their own reference values; they do not show that the industries began with equal output or revenue.
Likewise, changing the reference year used to display an index is not necessarily the same operation as changing the weights or methodology used to construct it. Check the publisher’s explanation when a series is revised rather than assuming every altered chart is a simple rescaling.
Before sharing an indexed chart, keep three labels with it: the measure, the reference period and the date range. Then describe the movement from a named starting point. “Up 10% from last year” gives the reader a clearer claim to check than “the index rose twelve” on its own.