Some axis codes don't have a string representation but should be accepted anyway
https://lists.freedesktop.org/archives/input-tools/2016-April/001287.html
commit 4943f2895fa8eac08eee63c1df091d7d63bcc3f4 Author: Peter Hutterer <peter.hutterer@who-t.net> Date: Fri Apr 22 10:10:39 2016 +1000 tools: allow numeric axis values in tweak-device
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