This is not the way Pi-Hole is configured. The dashboard shows the most previous running 24 hours of data (Queries over last 24 hours), and does not reset at midnight. The fluctuation you are seeing is where a previous high or low activity period rolls out of the 24 hour window and is replaced with more recent data.
A relatively extreme example of this is shown in the screen snap below. This Pi-Hole serves two clients, one of which is normally sleeping and the other which was turned off last night. There is zero activity shown for about 9 hours, and this is reflected in the total. As the high activity occurs this morning, the low activity from yesterday morning will be replaced and the total queries will increase. Eventually, if the computer is left on, the zero period will exit to the left and the total queries will be at a maximum.
