Fix sparkline puzzle rating

[kraktus]

Puzzle rating graph was too flat due to this issue: fnando/sparkline#17

before:

after:

[ornicar]

I was actually thinking of removing it... the coloured list under the board almost does the same and is just more useful because it's clickable

[kraktus]

It doesn't add any information but it looks visually nice. I tried to add a tooltip (using onmousemove function provided by the library) with the rating of the player at that time, without success so far