I think it is automatically increasing itself.
No, this is not even limited only to RED interface.
all interface limit graph to 10Mb/s
Here I start download of ubuntu (download speed is 2.2 MB/s)
there is 2 attached JPGs
1. Chrome showing 2.2 MB/s download (around 17.5 Mb/s)
2. RED interface graph showing no download at all and max speed is 861 kB/s, max scale is 1.2 MB/s (around 10Mb/s)
just like MRTG when MaxBytes is set to 1250000