A fundamental tenet of Technical Analysis is that history tends to repeat itself. The repetitive nature of price movements is attributed to market psychology; in other words, market participants tend to provide a consistent reaction, the collective psychology of fear vs greed to similar market events. An analog chart takes a historical price pattern and overlays it onto a current emerging price pattern which may (or may not) follow the same path as the historical pattern.
Each Monday, I review monthly charts for the major currency pairs primarily to determine reversals in long term trends. In another post, the GBPUSD chart suggests further downside and April 2015 support is being pressured accompanied by falling RSI and MACD.