Making her eat what she takes is definitely a setup for health issues, physical and mental. The baby plate she might reasonably see as demeaning and controllling, and what you want is a child who makes good choices because she wants to make good choices. Charging her for food she wastes is a not unreasonable consequence, and might work over time, depending on the size of her allowance and how important that is to her. Here's one more idea:
Would you consider attaching a "budget" with dollar cost to the foods you serve to give her a sense of the actual cost to your family? It wouldn't have to be precise, just a ballpark amount rounded to the nearest dollar, or even nearest quarter. You could give her coupons or monopoly money at the beginning of a meal (eventually make this for the whole week), with a value equal to just a bit more than she should be eating.
Have her pay for her portions as she serves herself, and if she has money left over, you could tally that toward a reward that she can 'earn' with enough savings in food not thrown away, maybe a trip to the ice cream shop, pizza parlor, or some special place like a movie or the zoo.
This comes close to bribery, which I don't believe in as a behavioral tool, but it would be strongly educational and may help break a habit based either in thoughtlessness or a power struggle. Once her habit changes, you could gradually phase it out.
I also strongly recommend the wonderful book How to Talk So Kids Will Listen, and Listen So Kids Will Talk, by Faber and Mazlish. This practical and accessible book is spilling over with real-life examples of how parents helped set the conditions for the child to address the problem herself. And though we don't usually think about children in these terms. they can be creative problem solvers. Plus, kids are more invested in solutions they think of themselves, and are more likely to work at making them a success.