I had the same problem, and solved it by making sure the OxyPlot.dll is in the same folder as OxyPlot.WindowsForms.dll (I copied it there).
See also: http://discussion.oxyplot.org/topics/698-winform-designer-problem/
Customer support service by UserEcho
I had the same problem, and solved it by making sure the OxyPlot.dll is in the same folder as OxyPlot.WindowsForms.dll (I copied it there).
See also: http://discussion.oxyplot.org/topics/698-winform-designer-problem/