+ left_label = ch->data_left + (ch->data_right - ch->data_left)/10.0;
+ right_label = ch->data_right - (ch->data_right - ch->data_left)/10.0;
+ centre_x = (ch->data_right + ch->data_left ) / 2.0;
+ centre_y = (ch->data_top + ch->data_bottom ) / 2.0;
+ radius = MIN (5.0 / 12.0 * (ch->data_top - ch->data_bottom),
+ 1.0 / 4.0 * (ch->data_right - ch->data_left));