Refactored graphing and improved performance for long-running tests

This release improves performance and reliability as follows:

• The sample rate is decreased to one sample per 10 seconds after 100 minutes to stay responsive in tests lasting many hours.
• The PNG image of the graph doesn't start writing until the background rendering is complete, because concurrency is hard.
• The output graph is scaled appropriately regardless of the user panning or scaling the live graph.
• A number pad is used instead of a generic keyboard for the calibration fields.