SUBROUTINE  PLTIL (X1,Y1,Z1,X2,Y2,Z2,PL)

C     [INTERRUPTED LINE]
C     DRAW A LINE FROM (X1,Y1) TO (X2,Y2) SHOWING ONLY THAT PORTION
C     WHERE Z IS POSITIVE, THIS REGION BEING DETERMINED BY LINEAR
C     INTERPOLATION FROM Z1 AND Z2.
C     PL IS THE PEN MOVEMENT SUBROUTINE, PERHAPS PLTCA.
C     [05-JAN-75]

      LOGICAL*1     P1,P2

      ZP(W1,W2)=W1-Z1*((W2-W1)/(Z2-Z1))

      P1=Z1.GE.0.0
      P2=Z2.GE.0.0
      IF (P1.EQ.P2) GO TO 10
      CALL PL (ZP(X1,X2),ZP(Y1,Y2),P1)
   10 CALL PL (X2,Y2,P2)
      RETURN

      END