xref

Sets the images's x coordinate axis. If set to a x axis id (e.g. "x" or "x2"), the x position refers to a x coordinate. If set to "paper", the x position refers to the distance from the left of the plotting area in normalized coordinates where "0" ("1") corresponds to the left (right). If set to a x axis ID followed by "domain" (separated by a space), the position behaves like for "paper", but refers to the distance in fractions of the domain length from the left of the domain of that axis: e.g., "x2 domain" refers to the domain of the second x axis and a x position of 0.5 refers to the point between the left and the right of the domain of the second x axis.