In the given search query 'index=security sourcetype=linux_secure | timechart count by action', the timechart command is used, which is designed to create a time-based chart. In Splunk, when using the timechart command, the x-axis is always populated by the _time field. This field represents the time at which the events occurred, making it the most logical and default choice for the x-axis in a time-based chart.