Fitting a straight line to a loglog plot data set and finding an arbitrary intercept value.

Hi Guys....

I'm struggling to find a way to fit a straightline to my dataset which is displayed on logarithmic axes.

I have a very sparce amount of data but they generally follow a pattern:

As you can see, above the data is displayed on loglog axes

     loglog(x,y)

Without going into too much unnecessary detail. I wish to fit a straight line to the data in order to find the x values when the fitted lines intercept with the blue lines (Wi*=0.002). An example is shown below:

I have tried using the polyfit command but to no avail. Any help on this problem would be much appreciated.

Many thanks,

Nath