circuit analysis with dc sweep graph

[n_sh]

hi everybody
this is my circuit graph ,can you explain for me what is caused the slope of my circuit graph?

AND this is Proteus file
http://uploadtak.com/images/e2969_111.rar

 

[Miguelrgp]

Please Vin Q6 and Q8 are undefinied, the DC sweep refears to ? I can't see it
You must set up values for Vin, Q6 and Q8 wich ones are fixed and wich one is variable. You know the axis of your graph but only you, plkease especify the axis.
Thanks And the DC sweep values.

 

[n_sh]

first,tank you for helping me.Vin Q6=-6 and Vin Q8=+6 and both of them are constant.
the DC sweep refer to Vin Q2,axis of graph is Vin Q2 when it's sweeping between -6 until +6.
i edited the picture for more information,and you can see my proteus file.