How do you decide the increment to use when numbering a graph?

Answer 1

Primarily by points that intercept the axis

Answer 2

Mostly, I want to make sure that the graph itself is meaningful -- you choose a range that encompasses the region of the graph that you want to view, then ensure that the increments in values on the X-Y axes make it easy to plot individual data points. So you don't necessarily want it to be in 100's if you have a lot of points within 1 unit of each other all the way up the graph, particularly if you need to view individual data points. For ease of graphing, you probably want to choose round numbers as well -- even if your data points are straight integers, unless there's a specific need to choose increments of 3 or 7, it just makes it more complicated to graph than if you chose increments of 10. If your numbers encompass a very large range of values, you have a few options -- I've seen graphs where they show the origin point as a reference, then include a "break" to indicate where you've removed an unimportant part of the graph, and the actual numbers start at the significant portion... or you could graph your data on a logarithmic scale so that the whole thing fits onto one graph even though the numbers encompass a large range.

Answer 3

You match the increments with the spread of the data.

Think of it this way, if you want to read household water pressure, which never goes to 60 psi, for instance, you would not use a 1000 psi guage.

A 100 psi guage still puts the operating range of the guage in the lower 1/3 of the needle span.

A 60 psi guage would be what you'd want. Matching increments to the data uses the same philosophy.
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Answer 4

Find your minimum and maximum values you will be plotting on the axis in question, look at the size of your graph paper, and figure what range of values you need to cover. Divide that range of values into however many sections you think may fit legibly on your graph paper.

If you need to do a range away from zero, e.g. on the x axis, you can always put a break indicator on your axis line like this:

------/| ./-------|----------|----------|----------|----------
........|/.......300......320......340.......360