Update CHANGELOG.md
Update CHANGELOG.md

Changes for #802, #801, and #535

NilesDavis/oxyplot,Isolocis/oxyplot,olegtarasov/oxyplot,DotNetDoctor/oxyplot,Kaplas80/oxyplot,Jofta/oxyplot,shoelzer/oxyplot,DotNetDoctor/oxyplot,ze-pequeno/oxyplot,HermanEldering/oxyplot,olegtarasov/oxyplot,objorke/oxyplot,shoelzer/oxyplot,Jonarw/oxyplot,HermanEldering/oxyplot,zur003/oxyplot,shoelzer/oxyplot,Kaplas80/oxyplot,oxyplot/oxyplot,objorke/oxyplot,objorke/oxyplot