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.
From the left side you can search your inventory of devices and service checks to pick out what you want to show on the graph. You can filter by just services or just devices, or everything. Clicking the name of a device will let you specify which metrics to plot.
If you have a cluster of servers that changes frequently so it doesn't make sense to statically choose the servers, you need to use an Elastic graph widget instead!
Selecting the name of the metric will plot it on the graph. You can go back and add any number of metrics from any device or service to overlay them. You can choose which axis to plot the metric on which is useful if the metric has a very different scale for the values:
You can set a name for the graph by clicking on "Graph Name". When you are done click the X in the top right and the graph will save and be plotted on the dashboard.