You see a data warning icon when the full set of data associated with a visualization isn't rendered or retrieved properly. If the full set of data can't be rendered or retrieved properly, then the visualization displays as much data as it can as per the fixed limit, and the remaining data or values are truncated or not displayed.
Next to the title of a visualization that has a data issue.
When you hover over the warning icon, you see a message that includes text such as the following:
Data sampling was applied due to the large quantity of data. Please filter your data. The limit of 500 categories was exceeded.
You see the warning icon associated with the visualization until the data issue is resolved. The warning icon is displayed only in the visualization Canvas; it's not displayed in Presentation Mode or Insights.
On the Canvas tabs bar if any visualization on the Canvas page has the data warning.
By default, visualization warning icons aren't displayed; You can show or hide the warning icon beside the title of the visualization by clicking the icon on the Canvas tabs bar. The warning icon is only displayed if a Canvas includes a visualization with a data issue. If a visualization with a data issue is in multiple canvases, you see the icon in all those canvases.