It bugs me a little that the animation curve in the tween editor inspector has a fixed height of 20 pixels (or less?) and the width matches the editor so that could be 300 which gives it a very skewed aspect ratio and you can't actually make out the shape of the curve
I was playing with your widget Aren and I added some guilayout options to the EditorGUILayout.CurveField(..), see line ~30 of UITweenerEditor
// change to..
AnimationCurve curve = EditorGUILayout.CurveField("Animation Curve", tw.animationCurve, GUILayout.Height(50f), GUILayout.Width(175) );
(editor layout seems quite flakey - if you specify a width less than 100 it becomes zero, guess its a bug in unity)
see attachment