Resolution:

The final puzzle piece was in the QML. I had to set the max x, y, z axis values appropriately for a polar graph which differed from the original square-image values.

axisX.min: 0
axisY.min: 0
axisZ.min: 0
axisX.max: 360 // [Degrees]
axisY.max: 256 // Max data value
axisZ.max: 90 // Max calculated radius

Final c++ was:

float xo = x - 64.0f;
float zo = z - 64.0f;
float radius {0.0f};
float angle {0.0f};
radius = std::hypot(xo, zo);
angle = std::atan2(zo, xo) * (180/M_PI);
angle +=90;
angle = std::fmod(angle, 360);
if (angle < 0.0)
angle += 360;
row[x] = QVector3D(angle, value, radius);