In a Tableau line chart Tableau assigns null values with a value of 0 (zero) on the X axis. For example you’re trending this year and the previous years data for sales (or whatever measure) with the months on the X axis and the sales on the Y. If it’s mid year then future months won’t have any sales data as they haven’t yet happened. Usually it is preferable to remove those Null values to tidy the line chart.
The data source contains data up to May of the latest year. This date is important because in the line charts you see Tableau has included the future values – June to December. These dates don’t exist in the data source; they are Null, so Tableau assigns a 0 value.
How to remove the Null values from a line chart
To remove or hide the Null values, format the green pill on the Rows shelf. Right-click on the pill, select format and this will open the Format window.
Select the Pane part of the format window to remove the Null values from your line chart. In this section you will notice at the bottom it has the option ‘Special Values (eg. NULL). By default Null marks are set to Show. Select Hide in the Marks drop down and the Null values will no longer be on your chart.
For another technique at hiding nulls check this article.