First, make sure you’re in the edit mode of your dashboard.
Then, follow these steps:
1- Select the “Trend” widget from the custom widget menu
2- Add at least 2 metrics
3- You can change the graph type to display lines instead of columns, simply edit your widget
4- That should do the trick!
Look the animated GIF below (http://g.recordit.co/AX447XQp5b.gif)