Appropriate method for calculating negative returns on a trading strategy?

I have a cumulative profit/loss time series below for a trading strategy, what is the appropriate way to calculate the returns in percentage for such a series?

My issue is the appropriate calculation for when the strategy becomes negative and going forward. I can't take log of a negative number and I am not sure if the arithmetic calculation is appropriate. I would appreciate any suggestions. I am using matlab. Thanks

 1.0e+003 *

    5.2735
    4.3922
    3.1878
    3.6250
    3.6982
    5.3774
    5.5748

    5.0108
    1.0355
   -2.4639
   -4.6589
   -4.2990
   -3.8678
   -3.1051
   -4.5356
   -4.8130
   -8.9671

   -9.0438
   -7.2986
   -7.1849