Click on the image to view the “image” in high resolution. Since it is actually a vector, not a rasterized image, you can zoom in as much as you want.
The numbers inside the dots represent the lap in which that particular time was done. Drivers who did their laps later in the session will tend to have faster times. Keep that in mind when comparing the lap times done by different drivers.
If you do not see a driver in the plot, it is because he did not do a race simulation stint during the practice session.
If you see less laps than the number displayed above the driver’s name, then it is because the lap was slower than the maximum time represented in this chart.