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This blog post by Stephanie Evergreen for Evergreen Data provides guidance on using Excel to add a benchmark line to a graph.
The blog provides step-by-step instructions including screen shots of each stage of the process.
(Evergreen, 2014)
Sources
Evergreen, S., (2014), Adding a Benchmark Line to a Graph, Evergreen Data. Retrieved from: http://stephanieevergreen.com/adding-a-benchmark-line/
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