geom_ternary_gridlines: Draw gridlines on a ternary diagram
In aeggers/votevizr: Tools For Vizualizing Election Outcomes

Adds a layer to a ggplot object drawn in ternary coordinates.

geom_ternary_gridlines(
  at = (1:3)/4,
  line_overhang = 0.05,
  text_overhang = 0.1,
  alpha = 0.75,
  col = "darkgray",
  size = 2
)

`at`	A vector of the points at which the gridlines should be drawn.
`line_overhang`	By how much should the gridlines overhang the edge of the ternary triangle?
`text_overhang`	How far from the edge of the ternary triangle should the text labels be?
`alpha`	Opacity of gridlines.
`col`	Color of gridlines.
`size`	Size of text labels.