I came across these two videos by Austin Gerig of the Said Business School at Oxford. One of his specialities is the impact of HFT upon markets and the videos below represent the price movements of 40 large-cap US stocks in 1-minute interval without HFT and with HFT. You notice that with HFT there is a clustering or schooling of prices around a central point. Without, HFT prices seem to much more dispersed.