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You could make a new column in your table indicating whether each row is before or after the process change, and then launch the distribution platform including that new variable in the 'by' group (hint: use a formula to check if the data column is greater than a specific date). Or, you could use the distribution within graph builder and add the new variable as a grouping variable, similar to what the Species variable does in this chart:
@ih provides a good answer, however, I prefer using Box Plots in combination with Violin plots to give the true picture of what the pre and post distributions look like.