Add axis labels to plots

[alugowski]

Very useful comparisons! Thanks to you I integrated Dragonbox into my own project and saw a massive speedup.

One improvement would be to put labels on the plots. At first glace it's not clear what the axes are, and one is tempted to think "higher is better" and hence std::to_chars appears to be the winner. After more insight one assumes the y-axis is "average runtime in nanoseconds" and reaches the opposite conclusion.

The x-axes are still confounding me. I'm guessing the last plots' x-axes are run numbers? I've no idea about the first few.