title: Linked data selection points obscured for high density plots
body: <p>For linked brush selection plots with high density of data it’s often not possible to see selected points in a secondary chart very well (or at all) when the associated points are selected in the primary chart.    This is just because they are frequently obscured by the non-selected points.   This can even be seen to some degree in the Bindings, Selections and Conditions example plots in the Altair documentations.  I'm referring to points selected on plot X that highlight associated points on plot Y.</p>
  
  <p>I wonder if there is way to have the associated points be brought to the 'front' (top) automatically?   Or if the  opacity (or point size) can be set low for out of selection data so the linked selection points are more emphasized?     It seems like the alt.condition command didn’t accept these options.</p>