Does the dashed curve keep each leg's implied volatility fixed as the stock price changes? I'd put that assumption beside the chart, so I can tell what the slider is actually changing.
Good question, and yes. Each leg keeps its own implied volatility from its quote, so the skew between the strikes stays as quoted, and that volatility is held fixed as the stock price moves across the chart. The IV slider then scales every leg's volatility by the same multiple, which is how you model a volatility crush or spike. You're right that the chart should say so; we'll add a line beside it.