Include empty values in pivot chart

Include empty values in pivot chart

Just wondering the following.

I have created chart reports. The form it is based on a form with fields that can stay blank/empty. The chart ignores these fields.
For example 1000 records with Gender field: 200 "Male", 300 "Female" and 500 left empty. The chart makes 100% of only the Male (40%) and Female (60%) together, but I would like it to be relative to the total records, so 20% Male, 30% Female and 50% null/empty/unknown.

Is that possible?

(note the chart also created a value "Unknown" but that has no records, so not sure what it's doing there)