Bitty Data Logger 2.4.0 was just released for both iOS and Android. It adds one new feature, namely the ability to trigger calibration of the digital compass in a connected BBC micro:bit, on demand.
Capturing and charting magnetometer data is just one of the great things you can do with Bitty Data Logger. Magnetometers need constant love and attention though, if you like your magnetometer data to be accurate. Accurate data can only be obtained if you calibrate the compass in the environment you intend to use it in. On the micro:bit, the calibration procedure involves you responding to a "DRAW A CIRCLE" prompt which appears on the LED display by rotation the micro:bit in space, causing an LED light to be lit all the way around the edge of the display. Once you've lit all the edge LEDs, a smiley face appears, indicating that the calibration procedure has completed.
You can now trigger the magnetometer calibration procedure over Bluetooth from within the Bitty Data Logger settings screen. You need to be running a suitable hex file on your micro:bit of course, but luckily there's just what you need available for download from http://www.bittysoftware.com
Oh and Bitty Data Logger is no longer free of charge. Sorry about that. See the previous blog post on Bitty Controller for the reasons behind this.