Problem with trend line fitting

Hi all,

I came across this site after searching for many hours online to no avail, and I'm praying you guys can help. My problem is fitting a trend line to my data (hopefully attached). Now the data shown is my experimental data which I am trying to fit to a model. To test the data's conformity to this model a plot x/p vs x should result in a straight line (which it does). From this I can obtain further information about my data from the constants (gradient & y intercept). However plotting a linear trend line doesn't seem to result in a straight line (see attached). Could this be because the range of on my x axis is too large? A power trend line does fit the data but not the model. Am I using the wrong trend line, is it a problem with excel (doubtful I know!), could it be a problem with the logarithmic scales of the x and y axis?????

If anyone who can help or give insight I would be INCREDIBLY grateful.

Many thanks

PS Excel 2003