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.. allows constrained fits .. this is a GPL lib that is included into the source tree to avoid adding another painful deendency. .. for details of the lib please see: http://users.ics.forth.gr/~lourakis/levmar/
GoldenCheetah
About
GoldenCheetah is an open-source data analysis tool primarily written in C++ with Qt for cyclists and triathletes with support for training as well.
GoldenCheetah can connect with indoor trainers and cycling equipment such as cycling computers and power meters to import data.
In addition, GoldenCheetah can connect to cloud services.
It can then manipulate and view the data, as well as analyze it.
Installing
Golden Cheetah install and build instructions are documented for each platform;
INSTALL-WIN32 For building on Microsoft Windows
INSTALL-LINUX For building on Linux
INSTALL-MAC For building on Apple OS X
OSX: 
Windows:
Alternatively, official builds are available from http://www.goldencheetah.org
whilst the latest developer builds are available from https://github.com/GoldenCheetah/GoldenCheetah/releases