Use the line-graph image for a team delivery plan. Text exhibit: Line graph titled 'Total Delivery Plan Cost.' X-axis labeled 'Packages shipped' with points at 0, 20, 40, 60. Y-axis labeled 'Total cost ($)' from 0 to 230. Plot points: (0, 50), (20, 110), (40, 170), (60, 230). Add a small label near y-intercept: '$50 base fee.'. What cost behavior classification does the graph show?