Pass NA if you want to plot samples without grouping.
You can pass a character vector with one or several column names from ".meta"
to group your data before plotting. In this case you should provide ".meta".
You can pass a character vector that exactly matches the number of samples in
your data, each value should correspond to a sample's property. It will be used
to group data based on the values provided. Note that in this case you should
pass NA to ".meta".