Charlie-san,
Thanks for your comment.
To work for graphs that have a logarithmic scaling on the x-axis, change the locator2coordinate function below.
locator2coordinate[list_, sample_] :=
Module[{l1x1y, l2x, l2y, s1x, s1y, s2x, s2y, d, c, ysolve, xlist,
ylist},
{l1x, l1y} = list[[1]]; {l2x, l2y} = list[[2]];
{s1x, s1y} = sample[[1]]; {s2x, s2y} = sample[[2]];
ysolve = Solve[d*l1y + c == s1y && d*l2y + c == s2y, {d, c}];
{{d, c}} = {d, c} /. ysolve;
({Exp[(#[[1]] - l1x)/(l2x - l1x)*(Log[s2x] - Log[s1x]) + Log[s1x]],
d*#[[2]] + c} & /@ list) // Sort
]
This is my result of your example.
For your reference:
