After choosing an arbitrary length to represent one, you make tickmarks on each axis to represent this distance.
在选择一个任意长度来表示一个距离之后 在每个轴上做标记来表示这个距离。
英语百科
Check mark
A check mark, checkmark or tick is a mark (✓, ✔, etc.) used to indicate the concept "yes" (e.g. "yes; this has been verified", "yes; that is the correct answer", "yes; this has been completed", or "yes; this [item or option] applies to me"). The x mark is also sometimes used for this use (most notably on election ballot papers), but otherwise usually indicates "no", incorrectness, or failure.