The MultiSites graph generates premonth archive tables (blob and numeric).
Enabling/disabling the graph would save database space and even maybe increase access speed.
I don't use this graph at all. Tendencies are calculated within the respective dashboard widgets.
As you observed, the sparklines on the MultiSites view are the same as those that appear elsewhere (e.g., Visitors | Overview) There is no incremental increase in database usage by using the same graph in two different places -- i.e., the underlying summary data is not duplicated.
Re-opening since this is still a performance issue with many sites, and isn't addressed by #1077.