Opening plots should plot first move by white and first move by black

[MarkZH]

Plotting the first pair of moves may skew the plots toward unusual but repeated opening moves. If white's most common opening move is e4 but has a lot of different responses from black while h4 is always answered by Ng6, then that h4 move will rank higher than the more popular e4 due to splitting the count of e4 moves among lots of different responses.

Also, since plotting the counts of longer openings has not proved useful, get rid of the option to specify the length of the opening to consider. Just plot white's and black's opening move choices.