QwtPlot::canvasMap: margins fixed in case of disabled axes


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1973 29e17f8d-cf37-471e-a288-72cc03a76eda
QwtInterval::widthL added


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@1977 29e17f8d-cf37-471e-a288-72cc03a76eda