Create a plplot Qt Designer plugin.
Attached, please find the a file with the designer.pro, qtplplotplugin.h, qtplplotplugin.cpp and Makefiles.
Also, I have include the release binaries.
You will need to make sure plplotd.dll, qsastime.dll and qt.dll (this is the Plplot Qt driver are in your path). I simply copied these to $QTDIR\bin.
Then place qtplplotplugin.dll, qtplplotplugin.lib in $QTDIR\plugins\designer.
The module is loaded under "Display Widgets" with name "QtPLWidget"