How to display total RAM size of a server along with used RAM size in Graph panel?

Hai, I have configured Grafana alert for RAM utilization with the following metrics,

metric for RAM utilization : (node_filesystem_size_bytes - node_filesystem_avail_bytes)

I’m able to get current & average values using Legend Values, but I couldn’t get the total RAM value of the server using the total option in Legend Values.

Is there a way to get the total RAM size of the server also in the graph???
Pls help me with this … Thanks in advance

Grafana version: 5.4.5
Prometheus version: 2.16.0
Node exporter version: 0.18.1