I can help here.
When you first created this chart and toggled your parameter to false you likely saw a warning that stated that there were 'X nulls' in the view. Something like this:
If you click on this warning you get two options:
Filtering the data would make your chart blank as all months containing a null would be excluded. This is clearly not right for you.
Show data at the default position will convert all null values to a 0 showing a 0 line on your axis. This is what I believe has been applied and is causing the line to show on your chart.
If you do a quick rebuild and instead of left clicking the null warning - right click it. This will allow you to hide the indicator tidying up your visual without causing the 0 line to show.
With regard to the axis, I am not sure you can dynamically hide the axis marks when you choose not to show the line. I would be tempted to right click the axis and de-select 'Show Header'
and then use labelling to show min and max values for the line:
This would give the user a scale on the line but would remove the axis. I appreciate this is not a complete solution on the axis but believe this to be the best way to display the data.
do you have a sample workbook.
There are multiple ways of fixing this issue.
Using parameter to switch your marks so it is at the bottom or top,
using reference lines.
If you have something to share I can demonstrate the different possible methods on your workbook.
even If you recreate something similar on superstore data this would also help.
Thank you for your response. I thought I had right clicked, but I might be mistaken. I tried recreating the chart, but since it uses the same variables and filters, it did not give me the option to choose how I wanted to treat the Nulls. Is there any other way to view that option again or change the setting somewhere within Tableau / the XML?