Duration Error on Burn Chart

[Sally Neal]

I am trying produce a burn chart in google sheets to record time remaining on a project, which has to be formatted as 'duration' but any data over 24 hours plots incorrectly. It seems to be wrapping values over 24 hours back on themselves, so for example 30:00:00 plots at 06:00:00 (which is 30-24). So it seems to be treating the 'duration' as if they're numbers on a clock when I need them to work as normal numbers - i.e. go over 24 hours. 

Here's the Sheet...

https://docs.google.com/spreadsheets/d/1ktbJQbbrs1VW0QfAri0vU-PktXwv7LK1CMPb-OuoeGA/edit?usp=sharing

and a screenshot is attached

Can anyone help?

Thanks