I am getting message saying that 'mixing steps with time and mode number domains while creating an XY data object prohibited' when I want to plot field history output for displacement domain data.
Since a frequency calculation does not use a time variable as far as I know, you cannot plot some variable over time through it which is what you seem to attempt. You should simply restrict your history plot to the step with the time dependence.
A bit late for Raimy but might be useful for others who have come across the same error. Straight to the point, when you have the XY data from ODB field output window, click on active steps/frame then pick the necessary step only, untick the others.