quote:

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Originally posted by sub.rosa

Ok, my graph looks like this:

I need help to find x and y values for that intersection point of those two lines and mark it on axis (with dashed lines etc.). But I'm very new in Origin and don't know how most of operations works, so I would need plain explanation, please!

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Hi,

It seems you are trying bimodal linear fitting to your data. 
If so, you can fit the whole data, and can get the crossing 
point(Cx, Cy) at once by the following way. Here, you assume 
that two lines are defined as: 
  At X<=Cx  y = A1*x+B1
  At X>Cx   y = A2*x+B2

  - Parameters: A1,B1,A2,Cx
  - Function Form: Script
  - Fit Function: 
      If(x<=Cx) y=A1*x+B1; 
      else y=A2*x+(A1-A2)*Cx+B1
  - Derived Parameters: 
      Cy=A1*Cx+B1
      B2=(A1-A2)*Cx+B1

So, you can get the crossing point immediately from fitting 
result like below: 

 

--Hideo Fujii
  OriginLab