plt_cusum — plots the results of the cusum test
pltcusum(res,siz,out)
res = the results typed list of function dfbeta
siz= the chosen size dor the test (0.01, 0.05 or 0.1)
out = the symobolic name of the file where the results are printed (default: %io(2))
nothing (printed on a graphic window)