Obtain graph equation from an image

[DB_13699250]

Hi all, 

I am trying to obtain the equation of a line (p'/p=1.0, the top line) from this image of the graph. The graph is in log scale and I have no reference to the original source of the graph, hence have no equation for it. Is there anyway to use Mathcad to obtain an equation. I am using Mathcad prime 9. Thanks in advance.

[terryhendicott · Accepted solution]

[DB_13699250]

Thank you so much!

[Werner_E]

Here are my 2 Cents worth.

Of course it only covers the straight line for ratio=1