tracer for outliers

[PappasChristoforos]

Two points for consideration in the outlier selection/annotation:

1. consider bringing the tracer for the selected outliers as a first color key, since in case of many variables/groups, the tracer is now located at the very end and for new users it might not be so clear that there is a 'switch' to hide/show in the graph the identified outliers

2. In future versions, consider adding different tracers for the selected outliers of each group/variables. I am working with time series data from many sensors, and the tracer of some selected outliers from one time series overlap/hide/cover data points from other sensors/time series, making thus the data clearing proceeding a bit challenging, since a single tracer is currently used (and thus turned on/off in the plot).