imagine the waveform as a bitmap image, one pixel per sample
a DAC driving the inverting input of the comparator sets your Y coordinate
adjusting the phase of your sampling clock relative to the waveform sets your X coordinate
take a few thousand samples at each pixel to get an intensity-graded image
this directly measured the cumulative distribution function of the waveform - the probability the voltage is <V at a given time
the partial derivative wrt dV is the probability density function,
the probability the voltage is =V at that time which is your final eye pattern