Hide the answer when plotting a problem mid-tutoring

When a student asks for a graph during a problem they are still solving, plot the structure without the values they must derive. For a DFT amplitude question, drawing the tones at unit height showed cycle counts and aliasing without giving away A. Render the image and look at it before calling it done: a matplotlib legend with ncols sat on top of the curves here, fixed by extending the y-limit above the data.