MACD - Moving Average Convergence / Divergence
The MACD indicator was developed by Gerald Appel in the late 70s and is used to indicate both trends and momentum.
It is based on a MACD line calculated by subtracting the difference between a short term EMA of the Close Price and a longer term EMA of the Close Price. A signal line is then generated by applying an EMA to the MACD line. And finally a histogram shows the difference between the MACD Line and the Signal line.
The MACD line will be above 0 when the short term average is higher than the long term average and below 0 when it is lower. So the further away from 0 the more divergent the EMAs. An increasing MACD line indicates upward momentum and a decreasing line indicates downward momentum.
The most common use of MACD is to track crossovers between the Signal line and the MACD line in order to spot turns. Crossovers of the MACD line and the 0 line are also useful.
MACD Line: 12 day EMA - 26 day EMA (Blue)
Signal Line: 9 day EMA of MACD Line (Red)
MACD Histogram: MACD Line - Signal Line (Orchid)
Sufficient Data for Accuracy
The MACD is based on EMAs which are one of several indicators that include an element of prior data. As such a 26 day EMA based on 50 days of underlying data will be significantly different to a 26 day EMA based on 500 days of data. This site will always include enough data to ensure 'accuracy'.