I think you ran into a bug in graph builder which does prevents nesting a fourth continuous variable on the x axis. If you change weight to ordinal, drag it to 'nest', then change it back to continuous you should have your graph. Make sure Automatic Recalc (Red Triangle > Redo >Automatic Recalc) is turned on. See the animation below.
If this fixes your problem you might consider submitting this to firstname.lastname@example.org.