Take a look at the applet: ??

Here you see the graph of a function where $x$ takes values from $-2$ to $4$.
If you let $x$ increase with a set step length $h$, then you can draw up a table of the corresponding increments $\Delta y$ of the function value.

If your step length $h=1$ then the table will look like this:

The corresponding diagram of increments is shown within the same graph.
The increment of the function is the difference between the function value at $x$ and that at the previous step, $x-1$.
This is useful if you know the formula.
You can then use your graphing calculator and enter at Y1= the original formula for the function ant at Y2:
Y2=Y1(X)-Y1(X-1).
This will give you the table of increments at Y2.
The calculator, however, cannot make a diagram of increments such as you see in the graph above.