When two lines belong on one chart
An overlay helps only when its shared scale has a clear meaning.
The short answer
Use a shared axis for compatible quantities. If you normalise different measures, label the transformation. Be particularly careful with independent left and right axes: choosing their ranges can make unrelated movements appear visually aligned.
A worked example
A temperature series [10, 15, 20] and a sales series [100, 150, 200] have a similar shape but different units. Drawing them on the same unlabelled axis confuses values that are not directly comparable.
Your turn
For our two GBP series, compare the original prices and then their indices. Write down which information each transformation retains and which it removes.
What this does not tell you
An illustrative calculation does not explain causes or predict future results. Check the dates, units and missing observations before using the method elsewhere.
Related term
UnitSources & review
Original Charts Haven example. All stated numbers are illustrative and allow you to repeat the calculation.
Next lesson: Compare the scales