Heatmap renders data "outside" the timeline

I’ve been using a heatmap to track HDD temperatures in my server, but after moving my Prometheus container-stack to another server, the data is rendered incorrectly.
As seen in the image below, the top row renders partially “above” the viewable area.

I’m not quite sure what to do to get everything inside the heatmap again, apart from defining a Ymax (which I really don’t want to do).