Add a chart showing relative use of offsite time

[nwarmenh]

I imagine a second pie chart that shows what proportion of your offsite time is used:

• arriving late (but see Only count a student late when they are actually late #213 and "unexpectedly late" calculations on viewreports.php broken #214 for related issues for counting this)
• checking out early
• being absent
• going offsite mid-day

[Appleguy15]

@BadtzMaru This is your area of expertise. Have fun :)