In my Metric Viewer, the y scale is way too big for the data being displayed - as if it were zoomed out. What's going on?

The issue is with the stoplight threshold that is set for that particular metric. If you switch the viewer to the Stoplight View, you'll see the threshold is set very high compared to the data that is charted (see image below).

Change the threshold lower to adjust the y scale accordingly, unless that is the actual threshold you want.