Line 6, where you eval the symbolic expression, doesn't work. I assume you mean to substitute x1 into y? As written, it substitute x into y, but there's no variable named x.
If you change that line to:
the code works. It may not look like the plot command plotted the data, but if you zoom in on the range where x goes from 0 to 0.1 and y goes from -2.01 to -2 you'll see a line connecting the two points whose coordinates are in x1 and y1.