The standard cell potential is a thermodynamic property defined for a cell under standard conditions (concentrations of all species at 1 M , pressure at 1 bar, and a specific temperature).
For a Daniell cell, the reaction is .
The standard cell potential is given by .
Since depends only on the nature of the electrodes, the nature of the electrolytes, and the temperature, it remains constant over time as long as the temperature is constant.
While the actual cell potential changes as the reaction proceeds (due to changes in concentration according to the Nernst equation), the standard cell potential is a constant value.
Therefore, the plot of versus time is a horizontal line parallel to the time axis.