Well, taking what you said literally, you cannot. The plot is part of the scriptable Bivariate object. It cannot exist on its own. You might actually want something else. For example, you can get a copy of this object using the << Clone Box message, but the copy is no longer linked to the Bivariate platform or the data table. It is not interactive. You can get a reference to it using the << Report message to the platform and then send more messages to change or query the report layer.
dt = Current Data Table();
biv = dt << Bivariate( Y( :weight ), X( :height ) );
biv rep = biv << Report;