I first noticed the problem while carving two layer HDPE but it’s more obvious plotting with a pen.
The area is X-axis (000-350mm) / Y-axis (300-730mm) as seen in the photo–the center of the area is worst.
I tightened up everything today and it’s still producing this result. I’m not sure how such a specific area would be impacted while the remainder of the work area appears to be solid.
What Phil said…
I was totally forum ninja’d there… 
For my pen plotting I use a mount with a spring in it so that the pen maintains contact with the print media even with small variations in the flatness of the underlying material.
What mount do you use? A friend printed one for me that I found, though I can’t find the site for it anymore. I can incorporate a spring, too, but haven’t taken the time to do so.
I used a 3D printer to make one. image
