I would place them at the end of the lines...at the top or left side of the line because braille readers tend to either start from the top and go down or from the left and read right. If you place them where they are in print, they could get lost for the braille reader and interfer with readibility. There is already quite a bit of tactual noise with the grid lines, the axis lines and the numbers. With the labels placed outside the grid, it is "cleaner" and easier to follow the lines. If possible, I would also recommend making each line a different texture so it would be easier to track (one from another) and identify each line with it's lettered label.