It's just that I'm not sure how to graph the future :)
A valid point :)
Perhaps the opposite approach could also be considered? That is, metrics of projects that have failed and projects that are being rewritten in a different language. (True, there can be just as many non-language-related reasons for those things too, but hey.)
Another cool thing would be more comparisons relative to the same "field" of programming. Comparing C++ to PHP, for example, doesn't make much sense, as you couldn't use one for the other even if you wanted to. (Going out on a limb here by assuming that anyone would want
to use C++ or PHP for anything :P) Of course it can get somewhat complicated when looking at, for example, Java, where people have tried to make it do everything including web programming, which is why you can't just say "well, only look at PHP numbers compared to Perl numbers then."
Some ideas anyway. Godspeed!