#7 Chart generation should be more conservative

[jkraybill]

Right now, every run of the agent causes all reports to
re-generate their contents: both HTML and chart. This
is very inefficient, since charts that have data
recorded daily only are generated 288 times per day. It
would be preferable to set preferences for both
frequency of data gathering, and graph generation. At
the least, a DashboardReport should gather data and
generate reports on the same schedule.