A scatter plot is a very effective means of being able to visually determine the relationship between the rates of change in stock prices between two stocks. The following graphs the relationship of the daily percentage change in the closing price between MSFT and AAPL:
This gives us a very quick view of the overall correlation of the daily returns between the two stocks. Each dot represents a single day for both the stocks. Each dot is plotted along the vertical based on the percentage change for AAPL, and along the horizontal for MSFT.
If for every amount that AAPL changed in value, MSFT also changed an identically proportional amount that day, then all the dots would fall along a perfect vertical diagonal from the lower-left to upper-right section. In this case, the two variables would be perfectly correlated with a correlation...