Karl Ratzsch - 2014-12-15

A way to do it is of course this:

a=2; fit a "-" using 1:($1<2.5?$2:$1>3.5?$2:NaN) via a
1 1.2
2 1.1
3 999999
4 0.9
5 1.1
e

, excluding the offending range (here around 3) from the data.