You want to use as much of the graph as possible so that you can see any patterns.

Let's do the x-axis first.

1. Count the number of lines the axis has: count like moving on a board game, don't count the first line. On this graph there are 20 lines on the x-axis.
2. Determine the maximum value to be plotted on the x-axis. For this data the maximum temperature was 80 ^oC.
3. Set up a ratio of maximum data value to number of lines:

1. You probably do not want to label every line and there is really no use. So let's label every 4 lines, that would be 16 ^oC for every 4 lines. But our data for temperature is every 20 ^oC, so we probably want to label every 20 ^oC. In order to do this you would label every 5 lines. The idea is to label the axis quickly and conveniently without trial and error.
1. NOTE: The data used here is designed to work out in a nice reducible fraction. If your ratio was not reducible to whole numbers simply divide out to get a value, maybe it was 4.4274 ^oC for every 1 line. Simply round up till you find something easy to use in scaling your axis. I would choose 4.5 ^oC for every line here, or 9 ^oC for every 2 lines. I should be able to label fairly easily with those values.
2. Back to our example. Scale the axis:

Now do the same for the y-axis.