How do you want to split them?
Starting situation (both columns selected from column list and dropped at the same time to Y-axis):
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Option 1 (both columns selected, drop them to Y-axis while holding shift. You can get the same graph if you drop first one and then try where you should drop the second to split Y-axis like this)
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Option2 (stack your data first from Tables/Stack. Drop data to Y-axis and labeling to Y-grouping on right side):
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Option3 (still using stacked data. Instead of dropping label to Y-group, drop it to Page)
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-Jarmo