chart.Serie and chart.Chart both make extensive use of ostensibly private methods of chart.Reference like ._get_cache. This should probably be changed so that the values can be accessed publicly. Also it looks like caching is required but not implemented.
I propose that chart.Reference has properties for values and reference.
Also the chart.Serie attribute could be a property that returns the value of the associated reference.
I plan to add this to my fork but will start by adding tests for these classes.