OxyPlot.Wpf: fixed compiler warning
OxyPlot.Wpf: fixed compiler warning

freudenthal/oxyplot,Kaplas80/oxyplot,as-zhuravlev/oxyplot_wpf_fork,lynxkor/oxyplot,Mitch-Connor/oxyplot,freudenthal/oxyplot,as-zhuravlev/oxyplot_wpf_fork,svendu/oxyplot,GeertvanHorrik/oxyplot,svendu/oxyplot,DotNetDoctor/oxyplot,TheAlmightyBob/oxyplot,sevoku/oxyplot,shoelzer/oxyplot,objorke/oxyplot,freudenthal/oxyplot,shoelzer/oxyplot,br111an/oxyplot,objorke/oxyplot,Sbosanquet/oxyplot,mattleibow/oxyplot,sevoku/oxyplot,ze-pequeno/oxyplot,HermanEldering/oxyplot,GeertvanHorrik/oxyplot,bbqchickenrobot/oxyplot,mattleibow/oxyplot,objorke/oxyplot,bbqchickenrobot/oxyplot,br111an/oxyplot,shoelzer/oxyplot,Sbosanquet/oxyplot,BRER-TECH/oxyplot,Mitch-Connor/oxyplot,br111an/oxyplot,Mitch-Connor/oxyplot,svendu/oxyplot,H2ONaCl/oxyplot,lynxkor/oxyplot,GeertvanHorrik/oxyplot,oxyplot/oxyplot,zur003/oxyplot,TheAlmightyBob/oxyplot,jeremyiverson/oxyplot,NilesDavis/oxyplot,HermanEldering/oxyplot,olegtarasov/oxyplot,olegtarasov/oxyplot,Rustemt/oxyplot,Isolocis/oxyplot,Rustemt/oxyplot,jeremyiverson/oxyplot,BRER-TECH/oxyplot,H2ONaCl/oxyplot,Rustemt/oxyplot,H2ONaCl/oxyplot,jeremyiverson/oxyplot,TheAlmightyBob/oxyplot,Jofta/oxyplot,bbqchickenrobot/oxyplot,as-zhuravlev/oxyplot_wpf_fork,Jonarw/oxyplot,lynxkor/oxyplot,Sbosanquet/oxyplot,Kaplas80/oxyplot,DotNetDoctor/oxyplot