Structure Activity Relationships... LogP, LogD and drug activity

I am a final year PhD research student and i am assessing novel anti-allergy compounds. I have calculated the LogP (how lipophilic the compounds are) and LogD (lipophilicity at pH7.4 of compounds which ionize) values using marvinsketch. I have obtained the biological activity of these compounds and wish to determine if there is a relationship between the LogP (or LogD) and the activity. I have plotted all the drug activities (the log of the activities, read i should do that somewhere) (at one concentration, on the Y axis) against logP (or log D) (plotted as logP squared, read i should do this because its not linear, is this correct??) and have what seems to be a random distribution of points, i am using GraphPad Prism version 5.
I need help on what to do to determine if there is a relationship there!!! i know you cant just try and fit a line or do non linear regression without having a reason and i dont know what analysis i should choose!! How do i investigate if there is a relationship if i dont know what the relationship is like or if there is any at all???
I read somewhere to use principal components analysis and partial least squares regression but i have no idea what these are and i cant find them in prism!!
I would hugely appreciate any help, I am due to submit my thesis in 3 weeks and my supervisor has never done this before so has no idea!
Thank you,