Convert a set of line segment "join decisions" into a weighted adjacency matrix describing which map unit symbols touch.

`joinAdjacency(x, vars = c("l_musym", "r_musym"))`

- x
`data.frame`

or similar object, each row represents a single shared edge (typically`sf`

LINESTRING feature)- vars
a vector of two characters naming columns containing "left", and "right" map unit symbols

A weighted adjacency matrix is returned, suitable for plotting directly with `plotSoilRelationGraph()`

.