yty_graph: Graph the year-to-year correcltions between stats
In BillPetti/fgrapher:

Graph the year-to-year correcltions between stats

yty_graph(stat1, stat2 = stat1, playertype = "batter", startyear = 2000,
  endyear = 2016, minpa = 0, year.difference = ifelse(stat1 == stat2, 1,
  0), line = TRUE, r2 = TRUE, yx = ifelse(stat1 == stat2, TRUE, FALSE),
  highlight.player = "", highlight.year.1 = 2015, text.position = "top",
  scale.distance = 1, save = FALSE, path = getwd())

`stat1`	First statistic to correlate. Must be a metric available in the FanGraphs database.
`stat2`	Second statistic to correlate. Defaults to the same statistic as stat1
`playertype`	Either "batter" or "pitcher". Defaults to "batter".
`startyear`	First year to generate data from. Defaults to 2000.
`endyear`	Last year to generate data from. Defaults to 2016.
`minpa`	Minimum number of plate appearances for a player's season to be included. Defaults to 0.
`year.difference`	Number of years between the two years being correlated. Must be between 0 and 4. When set to 0, the correlation between 2 stats in the same year will be graphed. Defaults to 1 if stat1 equals stat2 and 0 otherwise.
`line`	Whether or not to include a trendline. Defaults to TRUE.
`r2`	Whether or not to include the r^2 of the fit. Defaults to TRUE. Can be TRUE even if line = FALSE.
`yx`	If TRUE, applies the trendline y = x instead of the fitted one from the regression. Generally should only be used if stat1 and stat2 are the same. Defaults to TRUE if stat1 equals stat2 and false otherwise.
`highlight.player`	A player to highlight on the graph. Defaults to no highlight.
`highlight.year.1`	The year of the player to highlight. Use the first year – for example, when trying to highlight a player's 2015-2016 seasons, input 2015. Defaults to 2015.
`text.position`	Where around the highlighted point to situate the text. Options are "top", "bottom", "left", "right", and "none".
`scale.distance`	How far away to put the text for the label for the highlight. Over 1 is farther away from the highlighted point; under 1 is closer. Defaults to 1.
`save`	Whether to save the graph. Defaults to FALSE
`path`	Where to save the graph. Defaults to the current working directory