when the measured bandwidth exceeds 7Gbps, the graph becomes inconsistent and presents values as if 7Gbps were subtracted from the equation. For example: If the measured value is 9Gbps, 2Gbps will be drawn. Is it possible to fix this?

what version of RouterOS are you using? im on 6.48.6 and graphic suffer that around 3gb+
Shoot ticket to support.
It’s not certain they read this forum for bug-reports.
And yes, they will also ask which version you’re running.
Something similar already mentioned in Dude v6/v7 - Known bugs.