Roger wrote:

The smoothing filter seems to be there to iron out the large year on
year variations and give a better idea of the trend at a glance.

To my mind this is the wrong approach - if you have data, then put the
statistical tools to work on it. Smoothing things out so you can see
what might seem to be a trend is IMVHO a very poor substitute for
getting the figures via an analysis - they'll tell you which way
things are going, and a whole lot more bedsides.