Curved Data Date - why? what? when? is it for?

Member for

16 years 3 months
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click on the crooked progress line icon and it wil be a straigh line. [[wysiwyg_imageupload:7096:]] 

Member for

12 years 5 months
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OK, not that hard to solve, it is the "Progress Line" which overlays the Data Date line.  Obviously I didn't use it much prior.  I kind of assumed it was something like that but here is a confusing aspect.  I don't have a Baseline attached to that schedule - so how can it be telling me how ahead or behind the dates are - meaning how can the line be crooked?