For me it worked as soon as I have set-up the printRect like:
NSRect printRect = NSZeroRect;
printRect.size.width = (printInfo.paperSize.width - printInfo.leftMargin - printInfo.rightMargin) * printInfo.scalingFactor;
printRect.size.height = (printInfo.paperSize.height - printInfo.topMargin - printInfo.bottomMargin) * printInfo.scalingFactor;
self.hostingView.printRect = printRect;
op = [NSPrintOperation printOperationWithView:self.hostingView printInfo:printInfo];
Note self.hostingView
refers to the CorePlot hostingView.