Jet2's 'Low Fare Finder' is almost identical to the system British Airways use on their website and have done so for quite some time now - so I would say if they are claiming to be the first in Europe, then they are wrong.
Monarch Scheduled also show the lowest prices for flights two or three days either side of the chosen dates, in both directions, on their website.
However, it is a bloody good idea to show a range of fares on one screen - one that I would like to see on more airline websites. It lets the passengers see at a glance which dates are cheapest without having to trawl through loads and loads of pages and start new searches each time.