"The following graph shows the numbers of both cars in the UK, and a shocking reminder that at the current rate of decline, there will be no 306 GTi-6 or Rallye left by 2017."
Disagree. As with all cars the number will decline over time. The fewer the cars available the slower the %decline. The graph will slowly level off and so there will still be plenty of GTi6's and Rallye's available in 2017. What the graph does show that in the last 10 years, since discontinuation of the 306, the number has halved roughly. And so in the next 10 years (2021) it's likely to half again, ie there is looking to be around 125 rallyes from the 500 made and around 1000 GTi6's from the 4500 made (all approx). Then in 2031 it will half again. roughly 60 rallyes and 500 GTi6's still on the roads.
All graphs usually have a trend and this graph shows that although both have taken the plunge, it's also showing it's levelling off slowly
You missed one important bit of information:
________________________________________Bye bye Sundance Kid