You’ve probably heard the rule of thumb that if R-squared is .7 or better then the trend line has a good fit to data and in general the higher R-squared is the better it is. However, there are several different types of correlation coefficients and each is appropriate under different conditions. A good approachable article on this is at the Stats with Cats blog. (Yes, that is the real name.) A good blog to start with at the site is,
Secrets of Good Correlations
< http://statswithcats.wordpress.com/2010/11/28/secrets-of-good-correlations/>
And yes, the pictures of cats with Negative Feline Correlation, No Feline Correlation, and Positive Feline Correlation are funny.
Positive Feline Correlation
You can display the R-squared value on the trend line in your chart by selecting the Display R-squared value on chart check box in the Format Trendline dialog box. This puts the R-squared value as text in a floating box. If you want to move it, just drag it. To delete it, select it, then delete.
Learn how a good correlation does not imply causation.
Learn how to create a trendline in a chart or TREND data in a worksheet.
Learn how to extend a TREND line to create a forecast.
Learn how to show or hide a TREND line under the check box control.