x1 = 3.0; x2=-2.5; x3=-1.5; x4=-0.5;  x5 =0.5;   x6 =1.5; x7 =2.5; x8 = 3.0;
y1=-3.0;  y2 =4.5;y3 =-1.5;y4 =-2.5 ;y5 =	3.5; y6 =5.5; y7 =2.5; y8 = -1.0;
m1 = (y2-y1)/(x2-x1); m2 = (y3-y2)/(x3-x2); m3 = (y4-y3)/(x4-x3);
m4 = (y5-y4)/(x5-x4); m5 = (y6-y5)/(x6-x5); m6 = (y7-y6)/(x7-x6);
m7 = (y8-y7)/(x8-x7);
P1(x) = y1 + m1*(x - x1); 
P2(x) = y2 + m2*(x - x2); 
P3(x) = y3 + m3*(x - x3); 
P4(x) = y4 + m4*(x - x4); 
P5(x) = y5 + m5*(x - x5); 
P6(x) = y6 + m6*(x - x6); 
P7(x) = y7 + m7*(x - x7); 
f(x) = (x<x1)? P1(x):\
(x<x2)? P2(x):\
(x<x3)?P3(x):\
(x<x4)?P4(x):\
(x < x5)?P5(x):\
(x<x6)?P6(x):P7(x);
set xrange [-3:3]
set yrange [-5:6]
plot f(x), 0
pause -2