Adding objects to price charts

It is not difficult to add any objects to the chart if we know their coordinates because all matplotlib methods always plot one array-like object versus another. So, basically, all we need to do to add any special objects to a chart is to calculate their position in the list, or the array along the X axis and the corresponding value along the Y axis.

Let’s consider a simple yet valuable example. In Chapter 3, FX Market Overview from a Developer’s Standpoint, we saw that price takers can only buy at the ask and sell at the bid. We also saw that a large order can move the price a few points (pips) up or down because it consumes the liquidity from several levels in the order book. So, we can assume with a good degree of confidence that if the best bid suddenly became greater than the best ask at the previous tick, then it was possibly a trace of a significant buy order. And it works vice versa – if we observe a plunge of the best...