Ling -
In OnCommand setup a "group" and then add all the aggregates of interest to that group. Then run reports against the group to get aggregated data for the group.
For the web page setup - rather than export as Excel, setup the report to actually save the graphic. You can then reference the saved graphic in your web pages. Establish a schedule in OnCommand to periodically update the graph on an appropriate interval.
As long as one of the built in graphs meets your need, this isn't really hard to get up and running. If there isn't a built in graph that meets your need, you'd then have to run the report to export as CSV or Excel or similar and then generate a graph from outside of OnCommand.
Bob