I think the parent basically sensed that you're not a trained mathematician and is trying to throw their middle-school math textbook at you.

The simplest definition of a "graph of a function" is that it's a representation of the points satisfying some underlying equality. Your plot isn't that. A more conventional name would be a heatmap: a plot of a function that takes two parameters - x and y coordinates - and then assigns a third value (color) to each.

I don't think the distinction is all that interesting. They're both function plots.