How do Charts work? Why is it empty?

Charts can be viewed from the tab bar or within a workout by tapping on the exercise name. When looking at your charts in Profile > Exercises, you can pin them to the Charts dashboard by tapping on the pin icon. 

Check out the tutorial video in Settings → Help and Support → "Watch Tutorial Video" or here on this video:  https://youtu.be/ByvaXMey8OU?t=2m6s