The compare function tool is used to compare the market movements of two or more different symbols simultaneously. Comparing multiple symbols simultaneously is done for a couple of reasons. For example, it is an easy way to visualize market health as a whole. The result is one single chart containing easy to read, pertinent information spanning the entire market. Another popular use for a comparison chart is comparing two companies within the same sector. For example, in the chart below, there is a comparison between two automobile companies; General Motors and Ford.
This is he same theory as the above example, but it narrows the snapshot down to a single sector instead of the whole market. It is important to note, that when you add a symbol to a chart, the vertical axis will change from price to percentage. The percentage shown is the price movement for whatever period is currently visible on the chart.
This is absolutely necessary. If you switch it back to price, the chart will collapse unless the price values of the symbols are very close to each other. Below is an example of a Google and Apple comparison chart when switched back to price. Therefore when you switch to price, both data series flatten out and the chart loses its effectiveness.