Sometimes there's invisible whitespace in the numbers:
eval Avg=replace(Avg," ","")
Had this same issue - numbers appear as text so don't calculate or plot, especially when extracted with REX at search.
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Get the Status Indicator app - it will allow you to do single value text with colors and icons. There are examples in the Dashboard Examples app by Splunk.
,A better way to go is to get the 'Status Indicator' app. Think of it as a super-powered single value visualization. I did the same thing with it and it includes the capacity to add an icon based on output.
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