The svg:x and svg:y attributes of chart:equation are underdefined. Specifically, they are only defined by reference to
"See §5.1.2 of [SVG]. For drawing shapes that have a non rectangular shape, the coordinate refers to the drawing shape's bounding box."

Note that 5.1.2 of [SVG} says:
"The x-axis coordinate of one corner of the rectangular region into which an embedded 'svg' element is placed."

What is missing is a specification of which corner and where the origin of the coordinate system is. Moreover, it seems make little sense to define the location of the equation relative to the bounding box of the regression curve itself but it would make more sense to be relative to the plot containing the regression curve.