What does the x-axis of a flame graph represent?

Why this is the answer

A flame graph sorts frames alphabetically precisely so identical stacks merge into wide blocks; left-to-right position carries no meaning at all. Reading it as time is the most common mistake, and it describes a flame chart — a real and useful visualisation from browser profilers, which is why the confusion persists. Call order runs vertically, bottom to top, not horizontally. Sorting by cost is the largest-frame-left variant some tools offer, but it is not the default and is not what the axis means.