Programming an interaction

edit

Perhaps the problem came from indentation, that I have had to adjust a bit. Try this:

pol = polygon([(0,0), (0,10), (10,10), (0,0)], fill=True, rgbcolor=(0.81,0.81,0.81)) 
g = plot(x, (0,10), color="blue")
@interact
def _(c=slider(0.01,1, step_size=0.2, default=.5), 
      inac=checkbox(True,"Domaine Inaccessible")):
    a = x.subs(solve(-c*x+6==x, x))
    h = plot(-c*x+6, (a,6), color="green")
    graph = h+g 
    if inac:
        graph += pol    
    show(graph, aspect_ratio=1, ymin=0,ymax=10)

By the way, the code failed if c=0, as I already pointed in other question, and the plot didn't keep proportions when checking or unchecking inac. I've modified the code to deal with these problems.