Steps:

• In your system, create a custom checkbox field "abc"
• Add a two dimensional chart to the dashboard. Let's say the JQL is "status=open" in the filter
• Choose X axis = "abc'
• Y axis, any field. Let's say it is Assignee
• You will get an X axis column named "None". In there it will show it has data

Error:

• When you click on the link of the "value" show in "none", you get the JQL like "status = open AND assignee = admin AND abc is EMPTY". It shows you there are 0 results which is different from what the chart displays.