Comet chart shows how a measure for one period compares with a previous period. The chart represents shape of a comet, hence the name. It usually has one to two Dimensions on Rows and at least one Measures on Columns.
Move Category, Sub-category to Rows and Sales to Columns
Drag Order Date to Filter shelf and filter out 2018 and 2019
Change Marks card to Line. Move Order Date (choose Year) to Path
Duplicate Order Date and move it to Size. This will expland the line chart and make it look like a comet
You should now have a view that looks like this:
Formatting the Color
Now we finished creating the chart. We still need to format its color to show the difference between year 2016 and 2017.
Create a calculated field:
Calculated Field: Coloring
Drag Coloring to Color. This calculation means taking SUM([Sales] of current period and subtract the SUM([Sales] of previous period.
You now should have a Comet Chart that looks like this: