isnan checks moved from color map to spectrogram


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@2114 29e17f8d-cf37-471e-a288-72cc03a76eda
spectrogram example improved


git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@2115 29e17f8d-cf37-471e-a288-72cc03a76eda
git-svn-id: svn://svn.code.sf.net/p/qwt/code/trunk@2116 29e17f8d-cf37-471e-a288-72cc03a76eda