Tableau gives you those zeros in the text chart because you have a dimensional grid there.
In the attached I simulated that behavior on your line chart. In fact, the dimensional grid is expanded to the full size because there are other sub categories that have data all the way to the end.
On your line chart, take "Chairs" out of the filter. You'll see the line for Copiers will go only to the extent of its last data in the data source.
Put the same filter on your text chart, and you'll see the same behavior.
Special note: If you change your MONTH pill on Columns to DISCRETE, you'll see the line for Copiers in the bottom half get extended all the way to December.
I was about to reply the same. When I try to change my Month Pill to discrete it did change to zero. Though i don't like it.:-)
Thank you very much :-)