Show postgres metric in dashboard chart

When I view the chart for one of my node.js apps in the Explorer (Summary) I get a nice view of Node.js / Response / Postgres / Web external.

But when I add this app to the Dashboard, and show as a line chart, I see slightly different set of metrics: Response / Request queuing / Externals / GC / Application.

I think that Externals in the dashboard equates to Web External in the Explorer Summary.

How do I see the Postgres metric in the dashboard too? This is an important metric for me.

Thanks.