I'd be glad to provide some information about the sales graph shown in the Dashboard.
In QuickBooks, not all sales transactions are displayed on the dashboard. QuickBooks only displays the Invoices and Sales Receipts to calculate the sales amount on the Dashboard. Journal Entries posted to sales account, sales tax, and other income transactions like bank deposits will not be displayed.
Additionally, the data being shown there is based on the specified period. Let's double-check the amount shown on your sales graph. You'll need to first make sure you filter its report dates correctly by clicking the This month drop-down arrow.
Once done, run the Sales by Customer Summary report to check the details of your Sales graph. Here's how you can open the report: