Thanks for your responses. Unfortiunately I'm not able to share the app.
Marwen - I have implemented your expression and it's looking a lot closer now, as per the below screenshot. I've amended the background data slightly which is why the figures don't quite match my original screenshot, but the rankings almost seem to be working. The only thing not quite right is that the rankings seem to be null for any countries where there is no volume shipped in the selected month (the volumes/rankings are based on year to date based on selected year and month, as per my original post). I'll try and figure this out now, unless anyone has any pointers?