The way this works is you look at the first number (which is 1 in this case), you say it out loud ("one 1"), and that becomes the second number in the sequence (11). Then we rinse and repeat--we look at 11, say it out loud ("two 1's"), and that becomes the next number in the sequence (21).
Here are the first ten:
The visualization you are looking at above is sequence number 25. The coordinates start at (0,0). Each number in the sequence is multiplied by (0,1) Up, (-1,0) left, (1,0) right, and (0,-1) down in turn. All of the coordinates are converted to absolute values so they trend from bottom left to top right on the canvas.
You can view the block here: http://bl.ocks.org/ngopal/9864113