Its value gives the spacing between the bottom edge of the title and the top edge of a bounding box containing all the other parts of the annotated grid. Positive values cause the title to be drawn outside the box, while negative values cause it to be drawn inside.
The TitleGap value should be given as a fraction of the minimum
dimension of the plotting area, the default value being 0.05.
AST A Library for Handling World Coordinate Systems in Astronomy