You can have ticks (lines) that are located on the inside of the graph, on the outside or on both sides. For the x-axis "inside" means above the axis. For the y0-axis "inside" means on the right side of the axis, since in most cases it's located on the left side of the graph. For the y1-axis "inside" means on the left side of the axis, since in most cases it's located on the right side of the graph.