slope from semilog paper coordinates

I have data from semilog plot. x-axis in rad/sec (logrithmic scale) and y-axis in db. I want to find slope in db/octave. the data is as under:

(

x-axis (logrithmic) in rad/sec, y-axis (dB))
(11.4,-21.2916),(18, -28.9696), (24.35, -34.1729), (30.7, -38.1418)
(36.95,-41.3584),(43.2,-44.0662),(49.45.-46.4051),
(55.7,-48.464),(61.95,-50.3039),(68.15,-51.967)      
(74.4,-53.4838),(80.6,-54.879),(86.85,-56.1694)  
(93.05,-57.3712),(99.25,-58.4948)

Thanks in advance. I want to find the slope. by manual calculation, i got it -11.84 db/octave. Please suggest how to find in MATLAB