Found this error to happens all the time.
Open a new page (any size, Letter for example) on CorelX6, on right top corner of top ruller, make sure is in inches.
Make a single small horizontal line 0.1" long (hairline), anywhere within the page.
Go to Window menu on top, ToolBars, make sure Property Bar is selected.
With the line selected, on top left of the Corel, Property Bar, make x:0.0" and y:0.0", so the center of this little line will move EXACTLY to X:0, Y:0 of the Page.
At the right side where you entered X:0.0 and Y:0.0, you can enter <-> 0.1" and Vert: 0.0" to make the line exactly 0.1" horizontal.
With the line still selected, go to FILE, EXPORT, select PLT (HPGL), choose any name.plt (remember the folder), click "SELECT ONLY" and make sure the "DO NOT SHOW FILTER" is unselected.
Click EXPORT, the HPGL EXPORT screen will open.
On "PAGE" bottom select "Plotter Units: 10,000 per in"
Also, make sure " Plotter Origin: is " Bottom left"
On " ADVANCED" right click " NO WIDTH OR VELOCITY COMMANDS"
Now open the exported " name.plt" file from the folder you remember, with NOTEPAD or other.
You will have:
The first line (PU) is the LEFT of the horizontal line, PU-500 -10 already show the error.
The -500 means half of the 0.1" (the length of the line), since the center is at zero of the page.
The -10 is the vertical (Y) value is WRONG, and it must be ZERO since the horizontal line is at zero vertical of the page.
The second line is the RIGHT side of the horizontal line, and the X is correct but Y is wrong again.
It shows PD599 -10;, the 0.0500" is correct, 0.05" to the right from X=0.
But Y should be zero, and it shows -0.0010", wrong again.
I already made several tests, couldn't find anywhere on CorelX6 where you could increase resolution or something like that.
The correct should be:
No Matter where you put the line on the page, when exporting it will always reflect -1/1000 of an inch on the vertical (Y) axis.
Can someone please duplicate this test just to make sure about this findings?
I hope there was a patch or something to fix this issue.
I don't have X6.
When I do the test that you describe using X7, it works correctly for me.