defaults
clear
x=[1:19]
y=[1:10]//[9:1:-1]+5*ran(x)
f1='(a+b*x)*(x<=x0)+(c+d*x)*(x>=x0)+(a+b*x-c-d*x)*1.e5*(x=x0)'
f2='(a+b*x)*(x<=x0)+(c+d*x)*(x>x0)'
window 5
defaults
set plotsymbol -1
graph x y
scalar\fit a b c d
x0=10
fit y=1*f1
set plotsymbol 0
graph\overlay x[where(x<=x0)] (a+b*x)[where(x<=x0)]
graph\overlay x[where(x>=x0)] (c+d*x)[where(x>=x0)]
window 7
defaults
set plotsymbol -1
graph x y
destroy a b c d
scalar\fit a b c d
fit y=1*f2
set plotsymbol 0
graph\overlay x[where(x<=x0)] (a+b*x)[where(x<=x0)]
graph\overlay x[where(x>x0)] (c+d*x)[where(x>x0)]
replot\all