Answer:
the cost function is defined as a step function
Step-by-step explanation:
You want to know why a cost graph has steps.
Cost Steps
In the US, current first class postage rates for appropriately sized envelopes are 60¢ for the first ounce, and 24¢ for each additional ounce.
Envelopes below a certain weight cannot be mailed, and above that minimum (up to at most one ounce), the cost will be 60¢. That is, the first step is at cost=60¢, and extends from some minimum weight up to 1 ounce at that cost.
Above one ounce -- even very slightly above, the cost is 84¢. There is no weight where the cost is 61¢ or 62¢ or any other number between 60¢ and 84¢.
For the purpose of determining cost, the weight is rounded up to the next higher ounce. This guarantees that cost values will be a discrete set of values.
Continuous weights
The weight can take on any real value. This is reflected in the continuous horizontal line at each level of cost.
The hollow circle at 1 ounce on the second step means that the cost for one ounce is not the value of that second step. The filled circle indicates the cost for one ounce is the value of the first step. The hollow circle also indicates that weights arbitrarily close to 1 ounce, but greater, will cost the same as for 2 ounces.
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Additional comment
The minimum weight envelope that can be mailed first class will be a function of its size. Outside a certain size and weight range, the envelope will not be "machinable," so will incur an extra charge.
