How would you draw a black line with TikZ that shades to white/transparent radially (like this) between two coordinates?
Shading is possible for rectangles (TikZ documentation, Ch. 46), though I can not define the start and end coordinate as easily as with a line.

Update: Another approach was to create such an shaded rectangle was posted here.
.2pt. You can use20and.1ptinstead... and choose a better value fordraw opacity. – Paul Gaborit May 11 '13 at 17:19