Some database queries can return null values, especially where joins are used. By default, when Plotalot finds nulls in SQL results, it changes them to an empty string for string columns, or zero for numeric columns. It also shows a warning in the chart editor. Warning are not shown on the front end.
Plotalot determines the column types as described above before fixing nulls.
Sometimes you might want null values to be included in your chart data as actual null values, for example if you use the Google Charts InterpolateNulls option to calculate missing data points, or if you want to see actual gaps in your charts. In such cases you can set the Plotalot Component Option "Fix nulls" to "No".
If neither of these options is right for you, you can use the SQL IFNULL( ) function to return a specific value in place of a null value.