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Categorical attributes that can be used for the Pie chart would display after any data node from the Data card on the left side of the Data Viewer has been clicked. 

Simply drag and drop the attribute, Sample name as an example here, to the Add data in the middle panel The first row in the data will be displayed by default in the histogram and in this case, it is the histogram of the expression values for the gene A1BG (Figure 3).

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