plot_title(theme_element)Plot title
Parameters
theme_element : element_text
Notes
The default horizontal alignment for the title is center. However the title will be left aligned if and only if there is a subtitle and its horizontal alignment has not been set (so it defaults to the left).
The defaults ensure that, short titles are not awkwardly left-aligned, and that a title and a subtitle will not be awkwardly mis-aligned in the center or with different alignments.