Creating a Chart with Dynamic Inputs

I want to create a graph with two lines and a common, horizontal axis. I only know roughly how many points will be in the graph, so I want to use a dynamic input.

I created three dynamic, named ranges to hold the data. I inserted a chart. So far, so good.

The problem came when I tried to enter the named ranges for the data source. I typed the following text into the field:
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but when I clicked 'OK', I got the following error message:
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What am I doing wrong and how do I fix it?

The Excel file is attached.