Set the directory where to write the data. Gnuplot needs to reify the data before it can be plotted. By default, this is done by writing out data files into a temporary directory. This behavior can be restored by passing in `Some("".into())`. This can be set to `None`, in which case the data is written inline, without a temporary directory. Note that this has somewhat spotty support in gnuplot,
(&mut self, data_directory: Option<String>)
source not stored for this graph (policy: none)