When you come back to the existing chart from another data frame it will open it with the last step of the data transformation at the right-hand side, unless it is Pinned.
And if the last step doesn’t have the columns that were assigned to the chart, then the column assignment will be removed, which often would produce a blank.
We’re thinking of improving this by setting the ‘last viewed step’ to the chart so that you will be able to open the chart with the step that you selected for the chart the last time you visited. It’s kind of like ‘Pin’ except it automatically does it only when you move away from the chart view.