Compact REPL display for any object inheriting "pmx_element" – the
values returned by pmx_point(), pmx_line(), pmx_ribbon(),
pmx_errorbar(), pmx_trend(), pmx_style(), and pmx_color(). Shows
the element type as a banner and the set fields inline as
name = value, name = value.
Usage
# S3 method for class 'pmx_element'
print(x, ...)