The graph you are currently viewing (that is hidden by this help information) is showing 
the total strength of earthquakes that ocurred in each year.

Only earthquakes that are greater than or equal to magnitude 6.0 are
shown. It is often useful to limit the display to a certain 'class' of earthquakes to improve
the accuracy of trend analysis. Over the years, earthquake monitoring equipment has
become more prevalent and sensitive, leading to the ability to record more and more
earthquakes that might have previously gone unnoticed. However, this is more true for
smaller earthquakes, and less true for more significant earthquakes. Limiting the view to
higher magnitudes helps to correct for this possibility.

You can experiment with other ways of viewing this earthquake information by clicking on
the four different menu options to the left and top of the graph.

Quake Strength - You can use this option to limit the view to only the more significant
earthquakes, or to include even the smallest earthquakes in the graph.

Data to Graph - Change this to show different types of information. You can
experiment with this setting and then use the help button again to see an explanation
of each of the available options.

Graph by - Use this menu to see the summary of earthquakes by year or by month.

Graph Style - This menu item allows you to see either the raw information (first option),
earthquakes, or to choose between two different ways of examining the trend over time.