-ve amplitude value in wx fft sink spectrum

Respected members
I am new in GNU radio and i just want to share about one problem i came
across related to the WX FFT SINK spectrum i.e. how can i get +ve
values of the fft spectrum(signal amplitude)? The values i got were
always -ve (the spectrum is shown below the 0 db amplitude)whatever the
flow grapths were. This question might had been discussed earlier but
anyone plz suggest me what can be the solution. Which parameter should i
need to change?