yref

Sets the images's y coordinate axis. If set to a y axis id (e.g. "y" or "y2"), the y position refers to a y coordinate. If set to "paper", the y position refers to the distance from the bottom of the plotting area in normalized coordinates where "0" ("1") corresponds to the bottom (top). If set to a y 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 bottom of the domain of that axis: e.g., "y2 domain" refers to the domain of the second y axis and a y position of 0.5 refers to the point between the bottom and the top of the domain of the second y axis.