Null formatting

An additional aspect of formatting a field is specially formatting NULL values. When formatting a field, select the Pane tab and locate the Special Values section, as shown in the following screenshot:

Enter any text you would like to display in the pane (in the Text field) when the value of the field is null. You can also choose where marks should be displayed. The Marks drop-down  menu gives multiple options that define where and how the marks for null values should be drawn when an axis is being used. You have the following options:

• Show at Indicator results in a small indicator with the number of null values in the lower right of the view. You can click the indicator for options to filter the null values or show them at the default value. You can right-click the indicator to hide it.
• Show at Default Value displays a mark at the default location (usually 0).
• Hide (Connect Lines) does not place a mark for null values, but does connect lines between all non-null values.
• Hide (Break Lines) causes the line to break where there are gaps created by not showing the null values.

You can see these options in the following screenshot, with the location of two null values indicated by a gray band:

You'll notice that the preceding line charts have little circle markers at the location of each mark drawn in the view. When the mark type is a line, clicking on the color shelf opens a menu that gives options, for the markers. All mark types have standard options, such as color and transparency. Some mark types support additional options such as border and/or halo, as shown here:

The Hide (Break Lines) and the Show at Indicator options work well in the preceding screenshot because those two options do not obscure the null values. However, the formatting only helps with actual null values. These are records that exist in the data but have null, indicating that there is no value.

However, there are no records for some dates in December. In those cases, the value isn't null, as there aren't even records of data. Tableau still connects the lines across those missing days, potentially making it difficult to tell that there are gaps. In that case, consider enabling data densification (you can accomplish this by ensuring the Show Missing Dates option is selected from the drop-down menu of the DAY(Order Date) field on Columns and then adding certain table calculations such as Index() to the Detail option within the view). This causes Tableau to generate records of data for the missing dates and treat the values as null:

Or, as an alternative, use another visualization type such as a bar chart, which more clearly indicates the missing days: