The print-X-axis Function

X axis labels are much like Y axis labels, except that the tics are on a line above the numbers. Labels should look like this:

    |   |    |    |
    1   5   10   15

The first tic is under the first column of the graph and is preceded by several blank spaces. These spaces provide room in rows above for the Y axis labels. The second, third, fourth, and subsequent tics are all spaced equally, according to the value of X-axis-label-spacing.

The second row of the X axis consists of numbers, preceded by several blank spaces and also separated according to the value of the variable X-axis-label-spacing.

The value of the variable X-axis-label-spacing should itself be measured in units of symbol-width, since you may want to change the width of the symbols that you are using to print the body of the graph without changing the ways the graph is labeled.

The print-X-axis function is constructed in more or less the same fashion as the print-Y-axis function except that it has two lines: the line of tic marks and the numbers. We will write a separate function to print each line and then combine them within the print-X-axis function.

This is a three step process:

1. Write a function to print the X axis tic marks, print-X-axis-tic-line.
2. Write a function to print the X numbers, print-X-axis-numbered-line.
3. Write a function to print both lines, the print-X-axis function, using print-X-axis-tic-line and print-X-axis-numbered-line.

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