g_return_val_if_fail (pixel >= 0, -1);
n = tower_lookup (&a->pixel_tower, pixel, &start);
an = tower_data (n, struct axis_node, pixel_node);
g_return_val_if_fail (pixel >= 0, -1);
n = tower_lookup (&a->pixel_tower, pixel, &start);
an = tower_data (n, struct axis_node, pixel_node);