Now this one is a bit more difficult to understand.
By looking at the Britain line, we can see it's horizontal. Of course, Britain's productivity isn't constant over so many years: what's really happening is that Britain is being used as the benchmark *for each year*.
As a result, if France's productivity is rising but Britain's is rising faster, then France's index number will fall from year to year as Britain's remains at 100 because its the benchmark.
The Calculation Is:
France's Index Number for Year = France's Productivity for Year ÷ Britain's Productivity for Year X
This is a bit more difficult to give a metaphor for without complicating it even more, but I'll give it a try.
If two cars are racing car A and car B.
Car A is in front of car B.
Car B is moving faster than Car A.
As a result, Car B will get closer and closer to Car A, until they are next to each other.
From Car A's perspective, Car B is getting closer to it, even though Car A is still moving fast.
I hope this still helps, if the metaphor doesn't help then ignore it!