Thanks chromatic for explaining that graph. (not to me, but to others).

Care to look at

http://www.google.com/trends/explore#q=%22PHP%20tutorial%22

and

http://www.google.com/trends/explore#q=%22Python%20tutorial%22&cmpt=q

These graphs are perfectly consistent with observations on TIOBE - see the respective graphs there (google trends of tutorial searches denote "future interest", while TIOBE denotes "present interest")

If Google trends fall (in absolute numbers - see below), the language popularity will fall. If they remain constant, language popularity will grow. There is a - thin - margin of fall up to which present popularity will remain at least more or less constant. C is a good example for that. If my calculations are correct a factor of 12 in Google trends decrease is the threshold for that situation.

Now to your questions: The normalized Google trends graphs show a decrease by a factor of 25. Which means that we're good if the absolute number of searches on Google has grown 25 times since February 2004. Right?

Well nothing easier than that. Enter Google annual search statistics. 1st try http://www.statisticbrain.com/google-searches/ 2007-2011 we have a growth from 1,200,000,000/day to 4,717,000,000/day. My head says that's a factor of 4, my pocket calculator says ~3.93

Surely you want now to present 2004-2013 Google annual search statistics that will suggest a factor of 25. Be my guest.

Google Trends for "programming"

That seems to demonstrate PetaMem's point. "Programming" dropped to 25% of its maximum (the "factor of 4" PetaMem mentioned). Which makes "Perl tutorial"'s 20-fold fall a reflection of an approximate 5-fold relative decline in popularity.

But perhaps I misunderstood your point. That wouldn't be a big shock to me since you included zero words of explanation.

- tye