bug when z =! 0.

by the way is it that the codes to draw magnetic field lines is hard that the reason why i haven't seen much magnetic field lines in Ejs applet?
only magnetic field vectors is pretty common now :)

i being thinking i need to add magnetic field lines one day on all my other applets because the sg syllabus still mention it. or should the syllabus be changed to include field vectors to replace field lines?