You may wonder how big you can go with SexiGraf? It turns out you already got the answer in your question: BIG
Check out how many ESXi and VM you can monitor with a single SexiGraf appliance:
Yes, this is 968 ESX, 12405 VMs, 155 TB of RAM and 1.7 PB of storage over 18 vCenters. We only pushed up the appliance to 8 vCPU and 16GB of vRAM to support the load.
We even support high latency networks since one of the vCenters pulling, in this case, takes around 10 minutes to complete where the polling is every 5 minutes: