Are you interested in SignalScope on the Mac, or on the iPhone? Since this is the Mac forum, I will suggest that you download SignalScope and give it a try.
On the iPhone, you can measure volts with SignalScope Pro, or with SignalScope when you purchase the data acquisition upgrade (via in-app purchase). In either case, you'll need to perform a voltage calibration to ensure the accuracy of your measurements.
The time scale in SignalScope for Mac can be set as high as 2s/div, and on the iPhone it can be set as high as 500ms/div. At large time scales the signal does sweep across the screen. What it does not do, yet, is begin to scroll once the screen is full.