Plot an error indicating the direction of phenotype specificity.
x-coordinate of the arrow.
x-coordinate of the arrow end.
y-coordinate of the arrow.
y-coordinate of the arrow end.
x-coordinates of the labels.
y-coordinates of the labels.
Labels for each end of the arrow.
Transparency of the labels.
Size of the labels.
Size of the arrow.
Colour of the arrow.
ggplot2 object.
plot_arrow()