To make a graph of the Win Expectancy of a baseball game, use the graph a game tool.
This data is from MLB games from 1957-2015, including postseason games. All data is available at Retrosheet.
Source files also available on GitHub.
The information used here was obtained free of charge from and is copyrighted by Retrosheet. Interested parties may contact Retrosheet at 20 Sunset Rd., Newark, DE 19711.