WPF DrawingContext seems ignore SnapToDevicePixels

I'm drawing a chart by direct calls to DrawLine on the DrawingContext. Since I want to avoid any anti aliasing feature, I tryed to put the SnapToDevicePixels=true on the parent UIElement, but I still have anti-alias:

The project was an old OS project not written for WPF4, but I retarget it to the Framework4, can this be an issue too ?