Add entries to Graph Builder Legends?

[#1]

If  a user defined a marker or color for a specific row, this marker/color will not show up in the Legend of Graph Builder.

To make a manual Update to graph builder legend when using colour or mark by column 

is it possible to add additional entries to the Graph Builder legend?
... then change the marker/color manually and add a meaningful label.

[#2]

I don't think you can add extra items to legend which doesn't really have any relation to the columns. Create a column which takes row states (marker and color) into account and use that as color/overlay (or both)?

[#3]

Relation to column is OK, if necessary, one can edit the label and the marker manually.

e.g. 

To add some dummy entries for "name":

1)  add a Points Overlay and deselect all Variables but "Color = Name".
The idea: don't show any data point - just create new entries in the  legend for every "name".
[for all the other plots, deselect "Color = Name" to keep them as they are]

2) In the legend, adjust the marker/color to the settings from row states
... and deselect unwanted entries