Today Usman and I have pair-programmed. We improved the legend builder. 

Here is an example

Check this out:

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b := RTMondrian new.
b shape rectangle
width: [ :c | c numberOfVariables * 5 ];
height: #numberOfMethods.
b nodes: RTShape withAllSubclasses.
b edges connectFrom: #superclass.
b layout tree.
b normalizer
normalizeColor: #numberOfLinesOfCode using: { Color black. Color red }.
b build.

lb := RTLegendBuilder new.
lb view: b view.

lb addText: 'System complexity view'.
lb addRectanglePolymetricWidth: 'number of methods' height: 'Line of code' box: 'a Pharo class'.

lb addColorFadingFrom: Color red to: Color black text: 'Number of lines of code'.
lb build.

b
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Cheers,

Alexandre