xgraduate — axis graduation
[xi,xa,np1,np2,kMinr,kMaxr,ar]=xgraduate(xmi,xma)
xgraduate returns the axis graduations which are used by the plot
routines (with pretty print flag enabled). It returns an interval [xi,xa] which contains the
given interval [xmi,xma] and such that
xi= kMinr*10^ar, xa=kMaxr*10^ar and the interval can
be divided into np2 intervals and each interval is divided in
np1 sub-intervals.