When data is a matrix, it will automatically be transformed into a
long-form data frame, where each row represents a unique combination of
matrix indices and their corresponding values.
# S3 method for matrix
fortify_data_frame(data, ...)A data frame.
An object to be converted to a data frame.
Arguments passed to methods.
Other fortify_data_frame methods:
fortify_data_frame.character(),
fortify_data_frame.default(),
fortify_data_frame.numeric()