Can I change the x (horizontal) to distance instead of time?
Are you on the Garmin Connect web site or the app on your phone?
If you expand one of the graphs you'll see this:
Or from the main activity page:
Technically not a dropdown but...
Yes. Above the graphs there is a dropdown which by default is set to "over time". Select "over distance".
There is no drop down on the graphs that I'm looking at in Garmin Connect