To change the mode to piecewise or linear, click on the tabs found on the upper right hand corner of the interactive.
For a linear function:
To change the cost per unit, drag the purple line on the top graph (up or down).
Investigate the total cost for a given number of units by dragging the white sliders on the bottom graph.
For a piecewise function:
To change the cost per unit (top graph), drag the blue or red circles left, right, up, or down.
Investigate the total cost by dragging the white sliders on the bottom graph.