There are often situations where we have a textbox (e.g., say, a Maximum or Minimum for the y-axis on a chart), and we want to have it be able to accept a calculation, but it can't. Or, sometimes somebody has added another control, and you need to first change that control's value so that you an select as an input a caluclation. E.g., for many visualizations: Row Manipulation > Select rows to show... It'd be better if any textbox had a mode where a user could click something and turn it into a calculation.