Copy the calculation you just created. Now right click on the Rows and select New Calculation. Paste your calculation here, remove the RUNNING_SUM() part, and multiply by (-1) to compute the size of the change in sales. Here is my calculation:
(SUM([Sales])) - LOOKUP(ZN(SUM([Sales])), -1)*-1
Drag and drop this calculation onto Size on the Marks Card. The bars will fill up.
Now let's show the positive and negative change with color. While holding the Ctrl key, drag and drop the running total difference in the sales calculation from the Rows, and place it on the color Marks Card. Tableau will create a copy of your calculation and color your bars. Click on Color Marks Card and select Edit colors. Check the Stepped Color checkbox and reduce the number of steps to 2.
That's it! You now have a waterfall chart.
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