@Mac Dude Your advice gives me plenty to consider.

@tomleslie I am thankful to you for even an incomplete answer. It is disappointing that Maple staff have not replied to a few basic "how to" questions.

@Carl Love I've been away from my computer for a few days so haven't replied to you till now.

Your additional attention to my original question is much appreciated. It will be a couple more days before I can study it in depth and add to this reply.

I've spent some time studying your latest replay. It's difficult for me to understand so I'll return to it later.

@acer Thank you. Implicitplot now shows the correct image.

@Carl Love Thanks.Your solution is pleasantly simple.

However in my Maple 2020 the implicitplot command produces an empty plot.

Here is a modification of your solution which displays a poor version of your plot.

SolY := solve(eval(W, params), y):  # too long to display
NopsSolY:=nops([SolY]);
display(seq(plot(SolY[i], x = -1.2000 .. 1.2000), i = 1 .. NopsSolY), scaling = constrained);

@Kitonum Complete plots of the three Watt's curves are displayed in the revised, uploaded worksheet.

WattsCurve.mw

@Kitonum It seems there is no way to display the complete curves

@Rouben Rostamian

This worksheet, like all your previous replies to my Maple Primes questions, gives me valuable new insights which I greatly appreciate.  

@dharr Thank you for your enlightening answer. I will study it more thoroughly for additional insights.

@Rouben Rostamian  What you say makes sense. Then this is the first time I've read a math assertion in Wikipedia and other web sites that is not true.

@Carl Love Thanks for the hint, but that didn't work either.

Could there be a problem with the worksheet name? It is several words connected by underscores. 

@Mac Dude Thanks for pointing out that both Int... expressions describe the same plane.

Can you refer me to other examples of plotting of a procedure so I can learn more of this technique?

Supplementary question: Is there any way plot options can be selected in the plotted procedure and implemented in the plot command?

For example, each of the plotted part spheres could be displayed with its own color. 

@tomleslie Thank you. I was not aware of the int(Integrand... technique.

@dharr Your answer will help me solve a problem I'm working on

@Carl Love Thank you. I'll modify the integrals as you suggest