A matlab program to calculate the 3dB bandwidth from spectrum diagram.

I want to find the 3dB bandwidth of the figure. I hv files in .csv when the wavelength in Y axis and has scale in db...X axis has a scale in nm. one way is to find the maximum value at centre value and subtract with 3db.
& find cross-sections of the spectrum corresponding x1 and x2 coordinates. result bandwidth (X2-X1 nm).
Spectrum example as in attachment.