OptionStrat is capable of showing future profit and loss calculations on positions that are already established.
To input a trade that you have already opened, select the type of trade from the strategy menu (for example, a Long Call). You will then see the profit and loss charts for a position opened now, but you can update the price to show profit (or maybe loss 😞) that you have already incurred. Just click the option (the green or red tooltip that says something like "330C"), and click on the price on the popup. You can then type in a custom price and press enter (or Done on mobile) to save it.
Remember that if you change the strike or expiration, the price will be updated to the current price and you will need to input a custom price again. Options that are using a custom price will have a small blue dot next to them to remind you that they are not using the market price.
To clear custom prices, just click the reload button next to the stock price and all of the prices will be updated for you.