Take into account the discrete nature of fuel tanks.

[mueslo]

Although this would overestimate the needed fuel, since a good rocket design would ditch fuel tanks when empty.

[tavert]

You'd also ditch engines and/or change engine type as your mass or TWR requirement drops. Better to think of the plot as one stage at a time. You could eventually have the user input multiple stages and save their design as they go, recording for example if the upper stage engines are allowed to be used asparagus-style to reduce the effective TWR requirement for the lower stages.