The bar image map fails due to a check if the bar item point is valid. In the function BarItem.GetCoords there is a line
"if (!_points[i].IsInvalid3D)"
which fails if the PointPair.Z equals PointPair.Missing. Therefore all points that have an X and Y, but miss a Z, will fail. However, for an image map only X and Y are needed, thus I think the line should be changed to
"if (!_points[i].IsInvalid)".