Click the +Widget button in the top right corner of the dashboard to display the list of available widgets. From there, select 'Graph' to add a graph widget to your dashboard.
Once you have picked the "Graph" from the dashboard widget selection, a blank graph widget will appear on the page. Click the Cog in the top right then click on View/edit.
From the left side you can select a target for the series (a device or service). You can then choose a metric and a source (specific CPU core for example)
You can then hit save to display the series on the graph.
You can set a name for the graph by clicking name in bold (Used Memory in this case). When you are done click the X in the top right and the graph will save and be plotted on the dashboard.