I can't compare both libraries since I only use QCustomPlot (for a year now) but I believe my comment may be useful to other nevertheless.
Pros of QCustomPlot:
- Publicly availabe on Gitorious,
- Living user base willing to help out (including the autor himself),
- Openness to LGPL licensing (in exchange for a donation, in my case),
- Very clean and concise interface,
- Very easy to find a bug, if present (my bugfixes were rarely longer than one line)
- Easy to extend;
- Very hackable into whatever you'd like to smash out of a cartesian plotting library,
- Good documentation,
- Available in condensed form of 1 .h and one .cpp file - handy for quick integration or purpose-built one-widgeters,
- Very good performance (in all of my use cases),
- It's obvious that the author likes nice code and can produce one,
- Very well thought out road map (for a loong time to come).
Cons of QCustomPlot:
- Very strictly cartesian (read: no pie charts),
- In my case took some time to get the necessary grip of all things important (if one likes to implement extensions of comparable quality to the basis),
- Maybe a little slow evolution (as many other one-busy-man projects),
- Only usable in main thread (i. e. when you need to generate pictures in, say, multithreaded webserver, you need to run this library in the main thread, whatever that means for your implementation).
If LGPL-ish license is required, there may not be a free of charge option (depends on the consideration of the author). Given the quality and usability of the library, the donation my company gave in exchange for the LGPL license agreement was not undue.