Just moved the post - thanks for letting me know about that!
I’ve reduced the width of the row labels as much as possible and changed the font color to white, but the “gap” is still a bit too wide.
I think you'll be better off hiding the header. On each blue pill on the rows shelf, right-click and uncheck "Show header".
I also added a blank new line in the title for the Number of submissions worksheet to line up the rows with the Data Senders worksheet.
If you want to make the rows even closer, you could turn Data Senders worksheet into a floating dashboard element. If you click on Dat Senders in your current dashboard it will give you the element's size and position, which can you use for the floating element, moving it right a few pixels. ...
As a side note, you currently have rows setup to auto-size, which (to me) isn't ideal when there are significantly more or less rows than your default view.
Thanks for the very helpful and straightfwd answer - really appreciate the clarification and suggestions!