Use the image to answer the question.
An illustration shows a number line with arrows on both ends ranging from 1 to 13 in one unit increments. Dots are plotted above the number line to indicate frequency. There is 1 dot over 2; 2 dots over 7; 4 dots over 8; 4 dots over 9; 3 dots over 10; 4 dots over 11; 3 dots over 12; and 2 dots over 13. There are no dots over the numbers 1, 3, 4, 5, and 6.
What number is the outlier in this dot plot?