Denote the variable f1()not f1()and use floor()insteadround()
plot [0:16] [-8.5:8.5] f1(x) with lines, f1(floor(x)+0.0) with steps lt 2
also set many samples to keep the quantized graph aligned correctly:
set samples 1000
round() floor, "0,5 " ( 0,5 1,5, 1,5 2,5 ..) "1 ".

