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.
The box represents a traditional box plot, with the line in the middle representing the median time done by each driver. The top and bottom line respectively represent the 75% percentile and the 25% percentile.
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.
Interactive FP2 chart
As a new addition to this blog, I have created this interactive chart which will allow you to see how the FP2 session developed. It is better viewed on a big screen such as a computer monitor or tablet, but it should work for mobile too.
In this chart, the point sizes are representative of the lap times done by each driver. Bigger points represent slower laps, while smaller points represent faster laps. The square-shaped points are regular laps, while the circular-shaped dots represent laps in which a driver pitted. The outline of the dots is colored depending on the tire used in that particular lap (red = soft, yellow = medium and hard = white). The x-axis represents the session time as per the F1 live timing app. I know that the session is technically 1 hour but the app assumes the session starts approximately one hour earlier than the actual start.
You can hover over the points shown on the chart to see more information about the selected lap. If you want a hi-res image, just click here.