High-frequency trading is on the rise, and market research firm Nanex has created a chart that shows just how widespread the practice has become. The animated GIF shows the use of HFT machines for buying and selling stocks in the US from January 2007 to January 2012 (each color represents a different stock exchange), and clearly indicates a dramatic increase in the practice over the last few years. While these computer algorithms make it possible to buy and sell stocks at high speeds, they can also have unintended effects on the market — just last week Knight Capital Group experienced a "technology issue" that led to dramatic shifts in the prices of notable stocks like Citigroup and Bank of America, causing a temporary halt on the trading of those stocks.