Answer
See image.
.
Work Step by Step
Equate $f(x,y)=c$
for each value of c
$\left[\begin{array}{llll}
c=-3 & x+y-1=-3 & \Rightarrow & y=-x-2 \\
c=-2 & x+y-1=-2 & \Rightarrow & y=-x-1\\
c=-1 & x+y-1=-1 & \Rightarrow & y=-x\\
c=0 & x+y-1=0 & \Rightarrow & y=-x+1\\
c=1 & x+y-1=1 & \Rightarrow & y=-x+2\\
c=2 & x+y-1=2 & \Rightarrow & y=-x+3\\
c=3 & x+y-1=3 & \Rightarrow & y=-x+4
\end{array}\right]$
Lines with slope $-1$, with y-intercepts $-2,-1,...,4$
