I think this chart shows buy/sell orders right?
Right.
If it does, how come putting down a buy order below current price means an almost instant buy?
Because as long as your price isn't too low, your order will likely be matched by the next person to sell "at market"; eg, if there's already a bid order for
BTC2 @ $96, and you place an order for
BTC3 @ $95, all it takes is for one person to sell at least
BTC5 at market for you to get your coins. The only way this won't happen is if a lot of other buyers outbid you before that happens, or if there are no sellers.
How is this graph getting people's plans?
It's not. It only shows current limit orders. Market orders and future limit orders obviously cannot be predicted, and therefore are not shown.
