The reference to 'Microflow X' is a broken reference and should say ' MF_RetrieveValuesForChart' instead of X. Thank you for pointing that out.
In your sub microflow, can you ensure that the last 'change' object for the Chart object 'Adds' the reference to the new Dataset (NewDataset_2) instead of 'Set'? This is a many-to-many relationship and it could be possible that you are setting the association and therefore overwriting the existing association between the Chart object and 'NewDataset.'
I would also make sure that you are passing along the correct dataset object to your Sub microflo. NewDataset is the input parameter in the first SubMicroflow call and NewDataset_2 is the input parameter for the second. It could be you are passing the same value in twice.
And the instruction on https://docs.mendix.com/howto/extendability/visualize-data-using-the-chartsjs-module is updated! Nice.