LTSPICE - Plot power
Ok. I figured out the answer (accidentally).

The whole problem was that .plot command was generating an empty plot pane and I had to explicitly pick traces from the list. In the list there were only currents and voltages.

So to plot another expression involving currents and voltages do this:

1) Pick visible traces by clicking on icon and plot something.

2) On the top of the pane, right click the name of the quantity plotted (Cursor changes to hand icon there). For instance, if you plot V(pdrain) as in example, right click it (which appears at the top in color)

3) A dialogue box appears in which you can input the desired expression. I entered V(pdrain)*Id(M1) instead of V(pdrain)

If someone would like to answer or solve the problem such that entire thing can be done from .plot command only without explicitly picking up traces, please feel free to answer.