Link the observations and the panel with a triangle
mark_triangle(..., orientation = "plot", .element = NULL)<dyn-dots> A list of formulas, where each side
of the formula should be an integer or character index of the original
data, or a range_link() object defining the linked observations. Use
NULL to indicate no link on that side. You can also combine these by
wrapping them into a single list(). If only the left-hand side of the
formula exists, you can input it directly. For integer indices, wrap them
with I() to use the ordering from the layout. You can also use
waiver() to inherit values from the other group.
A single string, either "plot" or "observation",
indicating the base of the triangle.
An element_polygon() object. Vectorized fields will be
recycled to match the total number of groups, or you can wrap the element
with I() to recycle to match the drawing groups.
When orientation is "plot", the drawing groups typically correspond
to the number of observations.
When orientation is "observation", the drawing groups usually match
the defined groups, but will differ if the defined group of observations
is separated and cannot be linked with a single triangle. In this case,
the number of drawing groups will be larger than the number of defined
groups.