How to plot a dynamic line chart for mem_total vs mem_used?

I am trying to plot a line chart for mem_total vs mem_used for all my ec2s in a single widget. I need this chart to be dynamically updated (when the instance is terminated and new machine comes up it should be able to identify and update). Currently i am using metric explorer widget where I get mem_used and mem_total as separate line charts. I want to have mem_total vs mem_used in a single line chart