4PL curve fitting with gnuplot

gistfile1.txt

$ gnuplot -e 'set term dumb size 100, 30; set fit quiet; set xr [1000:20000]; f(x) = (a - d) / (1 + ((x / c) ** b)) + d; fit f(x) "acc.txt" u 1:2 via a, b, c, d; p "acc.txt" u 1:2 w l notit, f(x) w l notit'



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     |    +        +         +         +        +         +        +         +        +         +
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     *    +        +         +         +        +         +        +         +        +         +
  69 ++---+--------+---------+---------+--------+---------+--------+---------+--------+--------++
         2000     4000      6000      8000    10000     12000    14000     16000    18000     20000