Logical function plotting software in Linux

I want to plot function like this.

if $x > 6000$ plot function $y = 6000+ \frac{x}{15}$ otherwise plot y = $6000$

Suggest me any open source plotting software that has linux version. I have tried KmPlot, KAlgebra, kst they does not seem to has such option.

KAlgebra supports ? ... : notation but when I enter, it says invalid syntax. If you know how to draw this funciton in

-