@eokeefee The full displacement is then the slider height minus the circle height:
laf.registerFunction("drawLinearSlider", function(g, obj)
{
obj.drawOutside = true;
var a = obj.area;
var norm = obj.valueNormalized;
var radius = 15;
var cx = a[0] + (a[2] / 2);
var start = a[1] + radius;
var usefulDisplacement = a[3] - 2*radius;
var cy = start + usefulDisplacement * (1.0 - norm);
g.setColour(obj.itemColour1);
g.fillEllipse([cx - radius, cy - radius, radius * 2, radius * 2]);
});