Wang H, Joseph JA. (1999). Quantifying cellular oxidative stress by dichlorofluorescein assay using microplate reader. Free Radical Biology and Medicine, 27(5–6), 612–616.
In the case of any fluorescent, you excite it at a particular wavelength and take the reading at the emission wavelength. In the case of DCFDA, you will obtain the intensity (as a reading) at 522 nm. You can plot the fluorescence intensity (y-axis) in the histogram vs. your sample (x-axis).