Qwt is a graphics extension to the Qt GUI application framework from Trolltech AS of Norway. It provides a 2D plotting widget and more.
I would not recommend using this library since authors break API not only between major updates (5 -> 6), but also between minor updates (6.0.2 -> 6.1.0). Relying on such idiots typically costs a lot of time. Also, it is very stupid to use file naming scheme different from one of Qt. It makes client source code inconsistent (compare #include <QWidget> and #include <qwt_plot.h> instead of #include <QwtPlot>)