It is the Y-axis or an index line, depending on where it is in the chart.
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It is the horizontal line at the bottom of the chart.
The vertical line in a pedigree chart means the next generation so- MOM DAD + + + means vertical line + Son & Daughter
If you put a vertical ruler anywhere on the chart, it hits the chart line at most once.
A scatter plot, a line chart, a bar chart are some possible answers.
These are the axes.