Intrday Midpoint Curve

I'm assuming you want this on ACP. As far as I know, there are no tools on Stockcharts to create custom indicators. If you can't find an indicator already available on Stockcharts, you would need to find or write a compatible plug-in that does what you want. Unfortunately, I'm not familiar with that process so I can't help you any further in that direction

You could experiment with Price Channels, however. It's not exactly the same thing, but the mid-channel might approximate what you are looking for. You would have to fool around with the length, probably starting with the number of bars in a session for the time frame you are working in and working backward.