-4 5 setxrange
-1 1 setyrange
20 setxunit

traceOx

xticks
0 xtick
[0 1] {xmark} apply

/A{-2 0} def  
/C{-2 -0.3} def    
/B {3 0} def
/D{3 -0.3} def   
1.7 setlinewidth

[A B] (]-[) ligne

setTimesItalic
   (a) C dtext
   (b) D dtext