Directly Proportional Functions, in Depth

What is the constant variable in this directly proportional function? $$\begin{array}{|r|c|c|} \hline \text{Number of Shirts}& \text{Price} \\ \hline 1&13 \\ \hline 2&26\\ \hline 3&39\\ \hline 4&52 \\ \hline \end{array}$$