That’s a great comparison page you’ve put up.
I share your concern about pChart and it’s likely future. My personal preference would be to look at the library with the most developers and the most users.
What you haven’t included in your list is the various licences. Are they all GPL?
What steps do you think we could take to make the charting tool a little bit “pluggable”? If we want to future-proof ourselves, the best way is to make the elements of the script that deal with the chart library something that can be easily removed from the rest of the code. That way if we ever need to change libraries, it can be done without terrible damage to anything else.
I put up a quick comparison of different charting libraries choice in LS2 at http://docs.limesurvey.org/tiki-index.php?page=Comparison+of+different+charting+libraries&structure=LimeSurvey+Development
Please take a look and share your opinions. Other charting libraries which are not featured there are also welcomed.