You could change the type of the activity to cycling, or another type of activity that uses speed, and not pace. Other than changing the activity type, there is no way to customize it. At least if you realy mean the Garmin Connect Mobile app. Directly on the watch, you can usually customize it pretty well, but it depends on the model you use. If needed, check the user manual for your model, and look up the customisation of activity screens.
Well the watch does not show the speed diagram I want to see, ok Garmin Connect doesn't show it either (because it displays pace instead) but it could show it if the programmers weren't so annoyingly lazy.
