Macro Library lineutil
various line utility functions
function lineboundarycoord(xmin,xmax,ymin,ymax,m,b,[order])
looks for where the line crosses the graph boundary.
supply graph boundaries, and m and b of the line.
order can be an array of "right", "top", "left", "bottom" indicating order of search
returns array(x,y,loc), where loc is a best guess for where it would be good to put the label