Hi @tims
Those fields that you are using in your picture are to define flow variables for output, they are not for input. To set the axis limits, define a couple of flow variables that contain the desired axis limits and choose them from the pull-down menus on the left. You can do this with a Table Creator followed by a Table Row to Variable node. This will give you a red flow variable connector that you can drag to the upper left edge of the Line Plot node. I had a related problem once, see this suggestion by @HansS for the solution.
It is a pity that there is no way to enter values in this window directly.
Best
Aswin