Solid bar indicates that this satellite was used in the last position calculation. Open bar indicates that it wasn't used. The colors in the right hand side match this so that dimmed satellites were not used for the fix.

The bars themselves are colored according to the 'age' of the last measurement we got from the satellite. If we haven't heard anything for about 2 seconds the bar is dimmed.

Drawing below the baseline is possible, satellite signal strength seems to be a 'magic value' and all the data I saw the lowest value was around 30 while the highest was appx 54-56. So I subtracted 30 from the value to get it to nicely scale into the 24 pixels that are available.