Help with a line chart

Hi this is probably a super simple and straightforward query. I am trying to create a line chart showing how long specific incidents have been open for. So in the Y axis I have the 'Sum of days to repond' and in the X axis I have the refrences of the incidents I want to show how long have been open for. For the majority of incidents I have a unique reference number however there are a couple that have no refrence number so I've listed them as N/A. Is there a way for me to show each N/A as a separate refrence if that makes any sense or should I just give each one a unique reference number. Also is this the best way to show this data in the first place or should I use something else. I am basically trying to show how long an incident took to resolve and comparing that with the ideal amount of days it should have took. So simple queires should take x amount of days and complex x amount of days. I know this may sound like waffle but any help is mucho appreciated.