I couldn't open your twbx file because my version is lower.
As any kind of information, to see historical data you need a date field.
If you are nor comfortable about append data in excel, maybe you can load this in a database.
About gradient color, using natural way you just can configure step numbers and start an final color.
If you need something different, you'll need to calculate. Something like this:
If Sum(Sales) < 200000 then 'Low'
Elseif Sum(Sales) < 400000 then 'Medium'
Else 'Nice' End
Drag calculation on color card and set each collor by tag.