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
And yes, the pictures of cats with Negative Feline Correlation, No Feline Correlation, and Positive Feline Correlation are funny.
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 extend a TREND line to create a forecast.
Learn how to show or hide a TREND line under the check box control.