Can't add data to a field - custom visual

Hi All,

I'm trying to extend the 'enhanced scatter' visual from the gallery. I've managed to what I require and have added an additional measure to the data roles which allows for extra functionality. In this respect, all is well. The visual does what I want and updates etc.

However, for some reason when my new role has data in it, I cannot add data to another role. Specifically the 'legend'. This is a grouping type and is necessary to distinguish between different series. 

I have absolutely no idea why I'm unable to add data. I've tried to debug put I can't find an entry point into the underlying typescript. It's as if it's not being called.

What situation would cause this to happen? Can it be remedied?

To elaborate, when I try to add data to the legend role, what I'm dragging dissapears but the UI does not respond and doesn't change. This field works perfectly when I don't have data added to the role I created.

Help greatly appreciated.

Thanks,

Simon C