A payoff diagram with additional lines showing profit and loss today and at intermediate dates, not just at expiration.
The expiration line tells you the endgame; the "today" line tells you what actually happens to the account tomorrow. For calendar-spreads, diagonal-spreads and anything with meaningful vega, these can look almost nothing alike.
Good platforms also let you shift implied-volatility and see the curve move. That is the fastest way to learn which of your positions are really volatility trades wearing a directional costume.
Example: a 30/60-day XYZ $50 calendar bought for $0.85. The expiration line peaks sharply at $50. The today line is nearly flat and shifts up or down with IV: raise IV by 3 points and the whole position gains about $25 per spread without XYZ moving at all.
Original diagrams for the ideas on this page. Illustrative, not real market data.
The bid-ask spread. Buy orders sit below, sell orders above, and the gap between the best bid (50.01) and best ask (50.04) is the spread you pay to cross. Bar length shows the size resting at each price.Time decay of an option's value. The part of an option's price that is only time — its extrinsic value — drains away every day and must reach zero at expiry. The slide is gentle months out and steepest in the final weeks, which is what traders call theta.
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