Adding stuff

spoonful by spoonful

Add flour spoonful by spoonful, and the number of grams goes up in lockstep. That's entirely predictable because it's the same stuff that you're adding each time: the flour has a density.

Mass and volume are constrained, linked by a constant density. Spoon in sugar instead, and the mass increases by more for each spoonful, but still in lockstep because sugar has a greater density.

Change one, and you necessarily change the other. The impossibility of doing anything else is a discovery about the world. The line on the graph is one way of writing this; a mathematical relationship is another.

mass = density × volume

There is no clock running: time does not appear on the graph or in the relationship. You can't get around it by spooning faster.