Here is one way to do what you want:
currentBI = SelectedValue( BarIndex() );
newBI = IIf( BarIndex() == currentBI - 48, 1, 0 ); //use + if looking backward
Plot( C, "", colorPaleGreen, styleBar );
PlotShapes( newBI * shapeHollowCircle, colorRed, 0, H, 20 );