1. Select Analysis > Create Calculated Field
2. In the Calculated Field dialog box that opens, do the following, and then click OK:
   1. Name the calculated field. In this example, the calculated field is named "Line Size"
   2. In the formula field, create a calculation similar to the following:

      IF [Category] = "Furniture" THEN 10
      //the Furniture line will be ~10x bigger than the standard line
      
      ELSEIF [Category] = "Office Supplies" THEN 5
      //the Office Supplies line will be about half the size of the Furniture line
      //and ~5x bigger than the standard line
      
      ELSE 1 //standard line size
      END
      

3. Right-click [Line Size] in the left-hand data pane and select Default Properties > Aggregation > Minimum
4. Drag [Line Size] to Size on Marks card

Optional Formatting 

1. To change the size of all lines, click Size on the Marks card and adjust the slider  
2. To change the range of line sizes, do the following:
   1. Right-click the size legend and select Edit sizes...
   2. In the Edit Sizes dialog, adjust the Mark size range slider 
3. To make the thinner lines always appear over the thicker lines, do the following:
   1. Right-click [Category] on the Marks card and select Sort...
   2. In the Sort [Category] dialog, for Sort By select Field
   3. For Sort Order, select Ascending
   4. For Field Name, select Line Size
   5. For Aggregation, select Minimum