See attached solution.
Sheet 2 is an alternative solution using calculated field instead of SUM(Sales). The colour legend contains 'Null'. So, this legend may not be ideal if you want to display it on a dashboard.
So, I've created a dummy worksheet, 'Sheet 1 Legend', where Null has been excluded. You can add this worksheet to your dashboard, but minimize and hide it somewhere discreet while displaying the legend only.
Hope this helps.
Thank you so much for your time Okechukwu.
You're welcome Justin. Glad it helped.