Reference lines are a useful feature which can be used to compare, refer to, or measure against the data values displayed in a graph. Reference lines are often used to show whether data has met a certain target. For example, they are often used to see which product category has exceeded the target sale of x.
When working with a fixed target sale, click on the analytics tab and then reference line and set this to your target value by selecting 'custom value' and adding your target value.
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However, if you want to adjust the target value and show a reference line according to each selected target value, a parameter is needed.
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Click create new parameter in the reference line tab. Now you can set your parameter by a list, range or all your values.
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Now you should be able to change the reference line according to the parameter you have set.