# simplified double pendulum
c=cos(t1-t2)
f1=-sin(t1-t2)*t2p^2-2*g*sin(t1)/l
f2=-sin(t2-t1)*t1p^2-g*sin(t2)/l
t1'=t1p
t2'=t2p
t1p'=(f1-c*f2)/(2-c^2)-nu*t1p
t2p'=(2*f2-c*f1)/(2-c^2)-nu*t2p
par g=9.8,l=1,nu=0.001
init t1=2.5,t2=2
@ meth=qualrk,tol=1e-10,atol=1e-10,total=100,maxstor=100000
@ bound=100000
@ xhi=100,ylo=-20,yhi=20,yp=t2
pe=-g*l*(2*cos(t1)+cos(t2))
ke=.5*l*l*(2*t1p^2+t2p^2+2*cos(t1-t2)*t1p*t2p)
# Reality check - total energy should be conserved if nu=0
aux te=ke+pe
done

Run XPP with this file dp.ode
Initialconds Go
Window Fit  
Viewaxes Toon

In the Toon window 
File  choose dp.ani
In Toon window  - click Go  to see the pendulum

Back in XPP you can play with initial conditions, or friction (nu) 
etc  
Back in the toon window, after you simulate in XPP, click on Reset and then
Go