analyze: show generators on plot
analyze: show unit file loading on plot

This will add another color to the legend called "Loading unit files"
Like the generators it will mark a part of the systemd bar indicating
the time spent while loading unit files.
analyze: fix font size on plot

The font-size was missing a unit so they were ignored. This patch sets
the unit to 'px' and adjusts the sizes a bit as the text got very small.