If you want scalable graphics, you should export to SVG or PDF. Both are supported in the core library of OxyPlot!
If you implement a exporter based on TiffBitmapEncoder, try
http://msdn.microsoft.com/en-us/library/system.windows.media.imaging.tiffcompressoption(v=vs.110).aspx
If you implement a exporter based on TiffBitmapEncoder, try
TiffCompressOption.None: http://msdn.microsoft.com/en-us/library/system.windows.media.imaging.tiffcompressoption(v=vs.110).aspx