  The linear system $|D|$ of a divisor $D$ on a metric graph has the structure
of a cell complex. We introduce the anchor divisors and anchor cells in it -
they serve as the landmarks for us to compute the f-vector of the complex and
find all cells in the complex. A linear system can also be identified as a
tropical convex hull of rational functions. We compute its extremal generators
using the landmarks. We apply these methods to some examples - namely the
canonical linear systems of some small trivalent graphs.
