Google Sites is a great way to organize information. One of the benefits of using Google Sites is to use visuals to more clearly communicate. Sometimes, those visuals are charts. Google Sites makes it easy to add charts to your Sites page.
The Answer is Always a Spreadsheet
Start with Google Sheets
If your chart is based off of data then Google Sheets is probably the best place to start! Organize your data and use the chart wizard to create a chart visualization.