Here is a way to make it work.
You have to switch from using the Custom Legend (which will always show all the entries) to using the default Legend command that is populated using a hidden Pivot command to provide the legend entries.
Here is a summary of the steps:
1 – Delete the Custom Legend.
2 – Add the Legend command from the tool bar. Delete the Legend entry at the bottom of the expanded Points command, so that the legend at this point is blank.
Now for the Pivot command …
3 – Highlight all three columns. Click the Pivot command icon in the tool bar.
4 – Add the same mask you are using for the points into the Pivot command ( you can copy and paste the mask using the gear menus or you can create a group and put the mask at the group level. I personally recommend the second option as you can see more clearly what the mask is.
5 – On the Pivot command you will need to change Group bars to ‘By Column’ and change the Fill to a ‘Color Scheme’, as shown below.
6 – Check the Hide button on the top right of the Pivot command.
Now you should have the dynamic Legend and only showing the points.
If any of the steps are not clear please let us know!